I came across this about a month ago! If you look at an image of the supposed page from the ‘SOE Manual’ as originally captioned in rifleman.org, you will note an asterisk on the title ‘Sniper’s rifle with silencer*’. I ignored that * for years and the editor from rifleman.org can’t track down who originally sent the info But in the Intro to the original point of publication (Melton’s ‘OSS Weapons’ (1991)) it clearly states ‘ Using the [OSS] 1944 catalog as a base, I have updated it to include weapons .. that were being designed and produced up to the very end of the war.... The presence of an asterisk following the item’s name will indicate that this page has been added to update the original 1944 publication’. I have checked the 1945 OSS catalogue and it does not appear there. The book was written before the internet meant that illustrations could be copied without context far and wide. So far, there are 4-5 Aux Units patrols who reported using Win74 and only 2 with No.42 scope - possibly as a trial as one patrol then reported exchanging the No.42 temporarily with a No.32 (which proved too heavy). No evidence that this was a standard configuration and it does not appear in any known Aux Units record (where the only specific record of issue is to Winchester 69).
I do not know where the SOE link came from. Were surviving examples so unusual that it was thought only SOE would use it?!!! There is a distinct risk that Parker Hale made some up, at least, post war from war surplus for commercial resale in the late 1960s/1970s and many ended up in the USA. So if anyone has ever seen reference to the rifle with No.42 scope in an SOE document, or know of its use in the field then I would be very pleased to hear of it.