Vintage S&W Revolvers and Blue Book nomenclature

[Brians356]

In the Blue Book Of Gun Values (Thirty-sixth Edition) under S&W "Model 29 .44 Magnum (4 Screw)" (1957 to 1961 production) there are a few variants listed. For example "5 in. barrel (scarce, must letter)" (emphasis added).

What does "must letter" mean? I've scoured the text and footnotes, but cannot find any explanation, nor do I find this phrase under any other S&W models.
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[rogina]

In the Blue Book Of Gun Values (Thirty-sixth Edition) under S&W "Model 29 .44 Magnum (4 Screw)" (1957 to 1961 production) there are a few variants listed. For example "5 in. barrel (scarce, must letter)" (emphasis added).

What does "must letter" mean? I've scoured the text and footnotes, but cannot find any explanation, nor do I find this phrase under any other S&W models.
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A letter from Smith & Wesson.

 

[wagonman]

Roy Jinks of smith & wesson will reserch your revolver from their records and give you a letter for a fee giving information on your gun where and who first shipped to detailed description on barrel length finish and any special features on gun.

 

[Brians356]

Thanks, gents. That makes perfect sense, given the discussion of such a service elsewhere in the book.
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