Pic looks like a 3000 series Spicer. Gas deuces used the 3052 (direct 5th), non-turbo multifuel used the 3053 (direct 5th), and the turbo multifuel used the 3053A (overdrive 5th). Most of the multifuels were upgraded to the 3053A, and were tagged accordingly. Quick check, put tower in 4th gear (over to right and forward) and rotate input shaft one turn. Does output shaft turn less than one turn, or more than one turn. If more than one turn, overdrive 3053A. Easier with top cover removed, the diameter of the input shaft gear will be larger than the first gear on the mainshaft (4th gear normal position) which is now 5th and overdrive. If the input shaft gear is smaller than the 4th gear, the tranny is direct drive 5th. Most parts (other than 4th and 5th gears on main shaft, cluster shaft and input shaft) are interchangeable between all three, including case.
Bell housing is SAE #3, bolts directly to gas deuce, or to multifuel with #3 to #2 adapter ring. SAE #3 does not like clutches above 12" because of diameter of clutch cover. I have numbers for an available 12" Long style clutch that was used in a gas deuce if you need it. With my Isuzu 4BD1, I needed to space the tranny back 1 3/16" from the SAE#3 bell housing for proper depth on clutch and input shaft length.
They shift quite good for a tranny rated at 23K GVW, not as smooth as a NV4500, but certainly not harsh or hard.
Hope this helps.
By the way, my Spicer 3000 series service manual specs 30(cold service) to 50 wt. Motor oil or mineral oil for use in these trannys. It is dated late 1960's. Todays Road Ranger synthetic 50 wt is the present recommended lube. Most 3000 series in deuces burn up 2nd gear as it's rotational speed at hwy speeds is quite high. But military specs 80-90 wt gear oil which conflicts with Spicer. My choice is RR synthetic 50 wt.