I'd like to know if the T420 (not T420s) will work with an older Ultrabay HDD and tray from a T61 ThinkPad that I currently have. (P/N 26R9247).
Unfortunately, no the T61 ultrabay tray will not work on newer ThinkPads because the T61 used a PATA interface in the ultrabay while the newer systems use SATA or SATA II / III
The bay dimensions are the same, but the interface connectors are physically and electrically incompatible and so the older option opticals and HDD adapter bays will not fit. In addition to the old PATA adapter, there is a SATA, a SATA II and and a SATA III ultrabay tray. The SATA II and SATA III trays have different interface connectors than the SATA adapter and all three have plastic "keys" of different shapes that project from the rear of the adapter tray to prevent us in an incompatible system. 3rd party trays may not have these keys.
What you need is the UltraBay HDD adapter series 3. (PN 43N3412)