When I picked up my $100.00 "craigslist" purchased BMW this how I found the bike. The old tank appeared to be pretty rough but upon closer inspection no dents were evident. There was one nasty scratch on the right side, no cap or tool box cover. The bike was pretty much beyond hope as the restoration cost would have been way more than the bike was worth. The chassis went to a fellow needing parts and pieces.
This is the /5 or /6 German Police Tank I recently acquired. I've decided to go ahead and restore the tank and hold onto it as a spare. I tried to sell it on eBay but the prices are down right now. I'm seeing dented up standard touring tanks going for under $150.00 and decent ones going for under $200.00. I won't give this one away. I ran it a few weeks ago and it only went up to $175.00. I had a $300.00 reserve on it. So I'll just keep it. I already purchased a key for the tool box. I'll eventually purchase new BMW Roundels, knee pads and get the paint redone in Black. The tank is actually pretty nice now though the paint is marginal and there are a couple of flaws in the metal work.
These tanks are pretty rare in the U.S. They are more common in Europe since that is where they originated. I have never seen another one other than in photos. This touring tank was originally a 6.2 Gallon Tank but with the volume loss from the tool box it is probably closer to 5.75 to 6.0 Gallon now.
This is my bike with the Heinrich Fuel Tank. I am still not sure of the capacity but have narrowed it down some. I think it is considered an 8.2 gallon tank. The similar ones without the tool box are sometimes 9 gallon. The toolbox displaces some volume. It appears that Heinrich made many different varieties of tanks and the sizes varied from one tank to the next. Searching the web I have found at least a dozen different makers (from the 60's & 70's) of after market BMW Touring and sidecar tanks. Some where steel, some aluminum and some were fiber glass. I recently saw one fiber glass one advertised but the seller recommended some sort of relining for the tank as the Alcohol in the newer fuels would eat up the tank over time. I think I'll stick with the steel varieties.
The photos below come from "Johns Beemer Garage" web site. He has an extensive gallery of BMW motorcycle fuel tank examples.
This is a near identical Heinrich found on the web. This one was referred to as a 9 gallon unit. It seems that many tanks look alike but when you really study them you can find subtle minor differences. The angles at the knee indents look a tiny bit different on this one. They have more of a radius at the knee indent areas.
I included this photo of a /5 because of its' braking set up. It has a Hoske Tank I believe. The front brake unit is a Munch unit from the German "Munch Mammoth". This brake assembly is very rare and probably impossible to come by now days.
Here is a /5 Police Tank with the radio unit still in place. I've read that about half the police tanks that surface are missing the tool box cover. The radio unit is very heavy according to the owner of this tank.
This is one of maybe 50 Aluminum Heinrich tanks which were produced. When I found mine I had high hopes that it was an aluminum specimen but that was not to be. There probably aren't too many of these aluminum ones left in the world. They would be very expensive if you were lucky enough to find one. There may be some painted aluminum ones out there hiding under multiple layers of paint.
This was referred to as a "Heinrich Fuhrmann 3 Tank. It appears to be identical to mine.
This is a 12 gallon Heinrich Tank on a later model BMW. This is probably the largest tank Heinrich ever manufactured. It is big.
This Heinrich is on a /5 BMW and appears to be the same tank as mine. It is setting on a real neat little cafe bike. Notice how the engine cylinders are offset when looking down at them. The riders foot pegs are also offset to match the cylinders. Most non BMW riders probably don't notice this trait of the BMW airhead.
We made a very quick trip to Las Vegas today. Left at 7:00 AM and back to Yuma at 7:00 PM. Picked up this green gem from a guy in Vegas. I'm showing a night shot cause it doesn't look so good in the day light. The right side is also the "best" side. A previous owner had a left side crash which damaged the left side. The cylinder/head is missing on the left. From the looks of the left side muffler the old girl took a serious hit. The seller thought it was a 1972 model but the serial #'s tell me it is a 1973 model, although manufactured in September 1972.
Surprisingly the headlight assembly survived the crash as did a lot of other parts. The left side of the tank is beat up and the "toaster" panel is gone. This bike could be resurrected but the cost would be prohibitive. If I had been saving all the old parts I've had in the past year I could have easily repaired the bike. I just don't want to take on another major project / money pit. It would need a different engine, carbs, tires and a lot of misc. pieces to get it going. I would easily spend another $800.00 to turn it into a running bike. After the wreck it was involved in, likely the frame and/or forks could be bent too. So with that in mind it will go under the "knife" tomorrow and be dismembered. I found a pair of keys hanging from a wire and by chance they fit the fork lock and seat lock. Don't often find locks with keys. It has a nice aluminum fuel cap which will be used on the /6 police tank. The fuel cap is about all I need off the bike. I will save all the little misc. pieces and bolts. A previous owner was definitely a hack. It has a Suzuki (?) throttle control, rear tail light ingeniously grafted on and a home made trailer hitch also. The guy also drilled holes in the front turn signal housings and appeared to have bolted fairing / windshield mounts to the turn signals. That basically ruined the housings. He also has nice mini "ape hanger" handle bars installed too. Well, it will be a nice Ebay donor for sure! 
I purchased an orphan Luftmiester saddle tank a while back for use with the side hack. It was ugly with many layers of paint. I used some chemical stripper ( my first time) to remove the paint. Ends up there were many layers of paint with a base layer of some sort of HEAVY rubberized factory coating. It was a nightmare to remove. Spent hours to get it all off and not damage the aluminum underneath. There were stress / tool marks from the original stamping so had to take varying degrees of emery cloth to remove the stamping / tooling scars, then used progressively finer paper to polish it. It came out pretty good although any minor dings in the aluminum now show up. I much prefer the look of the raw aluminum. The tank won't fit on the BMW as it interferes with the kick start lever. I might mount it to the left side of the side car. It has proper height to gravity feed to the carbs and will add another gallon capacity in a pinch. We travel a lot of back roads and gas stations are sometimes far apart.
Now the next challenge if hunting down a proper fuel cap. First I'll try the IBMWR list and if that doesn't work I'll have to do some scrounging. Last resort will be having a machine shop fabricate one from billet. ($$$$)
