Are you sure that you have a SM420 transmission?
I have understood that the SM420 had synchromesh 2nd to 4th and I usually think of synchromesh as having helical, constant mesh gears rather than the straight cut gears in your photo.
Some online sources give the part number 590474 as being a 4 speed transmission as early as 1931 which feels early for a truck to have synchromesh.
Some characteristics of your photos resemble standard 4 speed of the CMP era but I don't find the part number 590474 in the CMP manuals I checked. The "retainer, clutch gear bearing" 591327 does appear in CMP manuals and is listed as 1938-46 in a Master Parts Catalog. (590474 doesn't appear in the same master parts catalog applying to 1929-48 so may be a later than '48 number).
I think that GM basically assigned part numbers sequentially so early parts have lower numbers, parts designed at the same period have similar numbers etc.
I hope I'm not making you as confused as I feel.
Looking at your photos, I wouldn't give up on the gears. I suspect a good wirebrushing or scotchbrite pad buff would make them usable (subject to inspection after cleanup). Some simple old mechanisms are remarkably tolerant of imperfections.
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