The TM ("toolroom mill") pulleys are usually fibre, whereas the quite similar TL ("toolroom lathe") pulleys are usually aluminum.
Four sheaves plus a two-speed motor gives eight speeds.
Sixteen speeds for the TL, as it has a backgear.
"Consequent pole" motors (six wires, single voltage, two speeds) are usually found on the TLs, and conventional motors (eighteen wires, dual voltage, two speeds) are usually found on the TMs.
(A UM is a TM with a rotating table).
"Consequent pole" motors have been seen on TMs, however.
The cabinet drive system of a mill is nearly the same as a lathe.
(HLV-H lathes still use some parts of the TL, and those parts are prefixed with "TL").
TM/UM, TL and split bed HCs all came from the same serial number sequence, from
1936 to the 1970s.
The list of serial numbers, by year, was posted by me to the Yahoo! Hardinge_Lathe group.
Home-built arbors seem to be the rule rather than the exception. They are 5C on the spindle end.