SCSW doesn't get any closer. Here's a thought, but it will cost you money. SCSW says that the 5906 is included in a series of 100 pre-production models, including the 3904, 3906, 4506, 5904, 5906, and 6906. The serial number range was AIP0001 thru AIP0100. I don't know how exact you have to be with model, etc. to get a proper letter, but I suspect people try to letter guns all the time and send in the wrong model number, barrel length, etc. If you absolutely have to know the exact date, pick a serial number and give it a shot . . .

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Wisdom comes thru fear . . .