Can I ask for some differential ID help please.

[aerotruk63]

I'm trying to part out a 4WD frame and running gear. I've been able to figure out the rest but the diff is giving me a headache.
Frame is 63-66 by 3820304 K10 & K20.
GM Corporate 12 Bolt
Has bolt on backing plates so 1965+
Casting 3838227 on passenger side of cast housing ( no listing in any of my parts books) . GM M on drivers side casting.
63 shop manual states date code under the casting passenger side, not there.
Where are the other date codes stamped for other years? Seen mention on the tube but where?
Thick yellow paint was applied at one time so lots of scrapping and sanding to find the codes.