Derk,
Your Binoprism No. 5 Mk IV monocular is unusual and the first one I've seen. This is what I've been able to uncover so far.
The Binoprism No. 5 Mk IV binocular was introduced November 1941, and the black crackle finish such as your monocular has was introduced 1942. In 1947 the color of the finish of the Binoprism No. 5 Mk V was changed from black to olive drab. Since there is no mention of No. 5 Mk IV's in that directive, I'm guessing the model was no longer manufactured by 1947. Does your monocular have coated lenses? If so, it was likely made 1945 or later. Your monocular was probably manufactured by Ross who made most of the Binoprism No. 5's, but there were other manufacturers such as Nottingham Instruments Ltd. Ross' 1939/40 catalog does list a 7X50 Porro II monocular called the "Stepmonite". Also, as of 1941 the British army manuals reference several monoculars but none as far as I can tell were Porro II's like yours. I'm surprised your glass doesn't have a crowsfoot marking, but I'm not positive all British military glasses had it. If it was used by the Canadians, it may not have had it either. I've seen the O.S. marking before but don't know what is means. I'll let you know if I find out.
That is a terrific REL you have. What are its optics like? Also, how do you locate Crown Assets sales?
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