PTO's are specific to transmissions, you are correct, once removed, hard to identify, always label once removed. Helical vs. straight cut gears, pitch of gears, depth of driven gear into trans., are requirements for matching.
Books for matches are hard to find, even PTO shops do not share. Easier from a business standpoint is to sell new PTO.
Last edited by EdPruss; Sat Sep 22 2018 01:15 PM.
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.