You are likely
Going to have to spring for a factory letter if want better info.
They are IMO some sort of special order. All of the early 5096 3rd gen pistol I have ever seen started with a T..TBA etc. you have a very nice set.
The SCSW calls out the 5906 product code only as Low Serial number. The notes on 4506 says that production likely starred at DVM001. (unconfirmed). But that product code is absent. 6906 no data at all. Any attemt to figure out what they really are, who they were given to or anything like that would be speculation on my part anyway. You say they came from Birmingham. Were they sold by David Carroll? He might have some info if you know him. Good luck and please keep us apprised if you learn more.

Edit: They are very eary production guns: It's specualative but likely Sw set yhose S/Ns aside for someone who ordered them specifically for a specific reason. We know there is a Model 39s with serial number of 39. It was special ordered for a LEO. So these may be something similiar.
Again a $100 for a factory letter for 1 will likely get you info for all 3.

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