I know this is a pretty old thread now, but I pulled out my TM751 to check the firmware version.
(For reference, for anyone taking a TM751 apart, be very careful when splitting the halves. The circuit board with the electronics will want to stay with the back half of the case, while the antenna is attached to the front half. The antenna has a VERY short wire connecting it, and it would be very easy to pull this wire clean off while trying to undo the bottom case snaps!) This TM751 has a date code sticker of 07F23, but no firmware sticker on the outside. On the inside, there is the main circuit board (brown), with a very small IC that reads: "EM78P153SNJ". There is also a small green circuit board attached to this, with the markings "=11245A, 26" on the circuit board, and a single IC marked "HiMARK, RX3310A-LF, AF 1586.1b"
I'm pretty sure that all of these are just IC part numbers or board numbers.... so it looks like not all revisions of these will have a firmware version indicated...