Photograph from 1941 - Gelatin Silver process

I came across a book from 1940 called Darkroom Handbook and Formulary, by Morris Germain, A.R.P.S., and in the back is a photo of a cat taped inside that was taken in November of 1941 by someone named Matthew E. Hopkins. It is black and white and with age, I assume, as acquired a slight sepia tone, but it also has a pearlescent/metallic reflection if the photograph faces any light source whatsoever. From what research I have done - and I know VERY little about film photography - it SEEMS to have undergone a "gelatin silver process". The size of the image is roughly 3.5" X 4.25", which seems unusual to me. Any help with what it is that I have in my possession would be greatly appreciated! Thanks!

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