Red posted while I was typing.... Nope, I can't explain it either.

C-1 would appear to be another example of Bucheimer's impish sense of humor, or the holster may be mis-marked.

I think it's safe to conclude that the cylinder pocket in your pictured holster is not made for a 2-shot derringer.

From the master list cobbled together from multiple sources:

1 = Derringer
1 = HS Hi Standard Derringer
1 = GER Derringer, German
15 = .25 Automatics, Standard Frame Size
15 = S&W Chief Special 2”

Bucheimer & B-C Concealer holster – 1970’s catalog list.
C – prefix added to usual Bucheimer 2 digit code.
No C-1 or C-15 on this list.

C11 = S&W Chief, 2”
C12 = S&W M&P, 2”
C13 = S&W Chief, 3”
C14 = S&WMDL 10, 4”
C34 = S&W Mdl. 15, 4”
C52 = S&W MDL. 19, 2½”
C52RU = Ruger Security Six 2 ¾”
C53 = S&W MDL. 19, 4”
C53RU = Ruger Security Six 4”
C78 = S&W MDL. 59
C79 = S&W MDL. 39
C42 = Colt Det., 2”
C43 = Colt Dt., 3”
C22 = Colt Diamondback, 2½”
C23 = Colt Diamondback, 4”
C54 = Colt O.P. MK Ill, 4”
C62 = Colt Python, 2½”
C64 = Colt Python, 4”
C74 = Colt Commander
C75 = Colt Gov’t.
C77 = Walther PPKS
C80 = Browning Hi Power
C81 = Sig Sauer 220 Series

01-604-16 FBI 3 inch model 13

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