Model_Version_FSBfreq.ROM ,
where 
Model - T91, T91MT or 901;
Version - BIOS version;
FSBfreq - FSB frequency in SHE SPM mode, Mhz.
CPU core frequency in HFM is 10 x FSBfreq for T91(MT) and 12 x FSBfreq for 901.

Example:
T91MT_0601_160.ROM is
BIOS v. 0601 build for T91MT, 1600 MHz in SPM.