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What does the 21/28 overstamp mean?
ETA: nvm just saw in the gunbroker pics. Is that more desirable than one marked 1921?
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The difference is the model. 21 vs.28
The overstamp guns started off as 21s which were all made by Colt and then at the behest of the military the rate of fire was slowed down (as to not waste ammunition).
I am not a Thompson expert and somebody may be able to offer more clarification, but the primary difference between a 21 and a 28 is the actuator was made heavier to slow the ROF. There are some other minor changes but the net effect was a slower ROF for a 28 vs a 21.
The original Colt 21s guns were "restamped" or "overstamped" when the 28 military upgrade took place. The "1" was then restamped to an "8" on the side of the receiver.
So when you buy a 21/28 Colt overstamp gun you are getting that original Colt Thompson receiver and all the quality that went with it, but at a reduced price to a pure unmolested Colt 21 which doesnt have its marking messed with or original parts removed and replaced.
1928s were then made by Savage and Auto Ordance "Bridgeport" for the military until they were supplanted by the M1 and M1A, both of which are a much more radical departure from the original design than the 21 to 28 was.