Nice find, my friend, and at a Beemer parts store to boot AND you got out of there for less than a hundred bucks! THAT is rare in itself! lmao......
I have looked at fuel cells myself for the Exec and think I found the one I want. It holds 3 gallons, would replace the passenger backrest and the 'plumbing' would be pretty easy to install. They also sell pads for the cell so a passenger could use it as a backrest as well. I'd then have a 350+ mile range which would be real nice sometimes.
I'd have to do a little research, though, on beefing up the area beneath where the passenger backrest is since gas weighs about 7 pounds a gallon and the tank would probably be another 5 pounds. Your solution, although not plumbed into the fuel system, is certainly a very attractive alternative.
And to christopher robbins, a fuel cell is basically an auxiliary fuel tank that many LD riders use to extend their range. Cells are typically 'plumbed into' the fuel system and controlled usually by a simple petcock so when you get low and there is no gas stations around or you just do not want to hassle with the stop at that point in time, you just turn the petcock and keep on riding. Some folks will install an electrical pump that will pump the gas from your aux tank to your OEM tank..it just depends on how much you are willing to spend on it.
If memory serves me correctly, IronButt rules mandate that no more than 11 gallons are onboard and that you HAVE to stop at least every 400 miles, proving that stop by a timed and dated gas receipt. They do this for safety reasons.
Morris Kruemke, riding a GoldWing posted the longest distance without stopping or touching a foot down with his fuel-laden Wing (not during an official IronButt Rally). He had nearly 35 extra gallons onboard that allowed him to cruise over 1200 miles without dabbing a toe to the turf. An amazing guy with a true Iron Butt!
PS- Nadam...drop tanks are used by aircraft, mainly fighters, to extend their range. When they are empty, they just 'drop them off' to save weight and gain airspeed.