I believe in have one of the final versions you mention here.A rough guess is that Sears sold about 11,000 of the final version 12" while Atlas sold around 7,000. So I'll add the highest known serial numbers of the two model groups.
Mine is a Craftsman Commercial 12
MODEL 101.28990
SERIAL NUMBER 105099
I have plastic gibs on the crosslide, compound, and milling attachment. The carriage has a steel gib.
I have purchased replacement plastic gibs from Causing which were black rather than white, like the originals, but still feel that I could gain some rigidity by using steel gibs. By the time the plastic ones are tight enough to get rid of any slop the hand wheels are hard to use as there is just too much friction in the ways at that point.
If there are any steel gibs still available I would be interested.