I can't even find a record of a gearbox number for a 512 TR, let alone know where it is. I found a record of a gearbox number on an F512 M, but I still don't know where it's located. I'd be just as interested as knowing the answer to your question. Anyone...?
On another thread it was posted the s/n is here: GEARBOX: On the back casting, lower left corner looking forward.