Both Wolfram and S&W used different size codes for target sighted revolvers, your holster was sized specifically for the M&P or model 10 revolver. Wolfram & S&W called this the FBI type holster, to my knowledge the FBI did not use this holster. Heiser introduced a holster in the late 1930's, the model 457 holster, that they stated was designed for the FBI. A version with the hammer protector, model 459, was introduced after the war.
I know that B. T. Crump had a contract in the 1950's to provide holsters for the FBI. A number of makers produced FBI type or style holsters.
Given the condition of your holster I would not use it if it were mine.
There are many similar holsters available in useable condition for reasonable prices. Just my opinion of course.
Regards,
turnerriver

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turnerriver