Brownies can be identified by the numbers stamped on the ID tag. 5xxx and 6xxx series are most useful for our size trucks. 7xxx and higher are so large, they need to on a semi tractor. The leading number, 5 or 6 in the above cases represents the Engine Torque ratings for the input engine. There are 3 and 4 speed ones. When searching for one, make sure and get the shifter mechanism from under the cab of the truck. None are synchronized, however they shift easily.

Ed

'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.