We went back to the tank today and attempted to find the serial number. Starting in the area Hanno pointed out, we found nothing, but above where the data plate would have been, traces of the number did appear.
Once located, I took a whole bunch of photos of the remains of the number: straight on, from above and the sides, with and without flash, with and without a light shining from side on. I’ll put the usable ones in this message and the following ones.
As you can see, the number appears to start with 21 but the last two digits are very hard to make out. They’re both “round” numbers, though: something like 60, 66, 69, 80, 86, 89, or maybe even 30, 36 or 39. I hope our collective recognition ability and/or tinkering with images can give us a definite answer
