If you mean the Saginaw 3 and 4 spds made from the 60's to 80's, the casting numbers may be meaningless. Over the years ,the interals may have been changed, all the gears are interchangable in sets.
I've worked on quite a few of these units, the 3 and 4 spds use the same parts, bearings,main shaft,seals,snap rings,syncros,only the actual gears and number of syncros is different.All the 3 spds have the smae side cover,same for the 4spd.The 3 spd cases have a different reverse idler shaft support that 4 spds,otherwiswe the two are identical,but small casting bumps vary .The tail shaft housings have slight differences in small casing and drilled details,this only affects installing the tranny in it's original application.
Generally, the circular grooves on the main shaft tell the ratios,no lines-2.85 1st gear,1 line-2.54,2 lines-3.11,3 lines,3.50.
Some say there are input shafts that we're marked,I've never seen one,but it's always best to check the ratio by rotating the shaft and counting turns etc.
