In my Blog dated 8th February I made reference to the unusual viewfinder used by Mr Herbert in the small photograph which I added. I am pleased to say that I have traced what I think is the finder in a copy of Leica Illustrated Guide 11 by the famous James L. Lager published by Morgan and Morgan in 1978.( Page 125-first printing)
The finder is lacking in any number or Code Word but resembles a squashed plastic cup formed into a rectangular shape at the front. The model illustrated has a cross wire and bead, possibly for aerial photography, but no reference is made to this part in a railway context and, quite frankly, the addition of the wire looks rather a home made adaptation unworthy of Leitz. It bears the usual Leitz trade mark and is engraved '5cm'. There are no optical parts. It is shoe mounted and no doubt could be replicated in plastic card if one really thought it a good idea.
Given the rarity and the fact it was never marketed no doubt this finder came over from Germany after the War, I have never seen one in the U.K., beyond this photograph.