I bought a can about 25 years ago in NH and I just discovered that I have misplaced it.

Do I have any reasonable next steps?

email the details (serial, make model, date, your info, etc) to

NFAFAX@atf.gov

They're very helpful.

A few months ago, my SOT guy messed up my email on a form 4. I emailed NFAFAX and they fixed it and emailed me my stamp.

it looks like that may be a bit more complicated than that looking at the address's discussed in the thread and your current location. Did you move the can legally via a 5320.20 when you moved? if not that is your first step. and if you don't have the numbers from the original approal that might be tricky. do you have a copy of the approved original F4 (or whatever form it moved on) if so then its a request to the ATF.