The BSA with frame number (and duplicated frame number on the engine) WM20.111490 was built under contract S/5209. This was a contract for 12.000 bikes (frame numbers 104818 - 116817). Unfortunately the late war factory ledgers no longer exist, but we know from the Receipt Cards that BSA started making this contract in early September 1944, at 2.000 bikes per month. This means that your bike must have left te factory in December 1944. The military census number would have been C5552190. I've added two pictures of bikes from the same contract, one is a bit earlier than yours, the other one a bit later. Here we can see that the early bike still has a long Acumen pillion seat, the later bike already has the short version.
email (option): wd.register@gmail.com
But there is more: your bike is also in the post war KeyCards, meaning that after the war it also served in the BAOR (British Army on the Rhine). From the KeyCards we know that the post war registration number was 92YC27, and that the bike was disposed off on July 31st 1957. Which fits the fact that your bike was first registered in 1958. Lex can do you a high-res scan of this KeyCard for a modest sum. I've also added two post war pictures with YC registered BSAs.
email (option): wd.register@gmail.com