As your engine number in the log book does not coincide with the fitted engine, a Vauxhall rebuild, it might be that this present engine was infact, a factory exchange unit that was fitted while in civvy use. Unless of course, it is painted in Eau-de-nil or Sky Blue colour. Vauxhall exchange engine colour was black.
It was quite common for engines and other assemblies to go back to the manufacturers on contractors, under Ministry of Supply or War Office contracts. Bedford, Humber, Daimler, Austin, etc can often be seen with makers rebuild plates, sometimes the plate is Min. of Supply with a number stamped in often beginning with "B", this would be a contractors ID number or Army Auxillary Workshop (also contractors). It still happens today.
Having been involved with vehicle engineering for the Forces, including rebuilding engines in their own workshops, I know the standards of rebuild and they are very high. Contractors have to adhere to the same standards, if they fail to meet them, then they are likely to lose out. In later years, even railway companies have been involved, rebuilding Land Rover 2.25 petrol engines. I personally have built many types of Bedford engines on a line, as well as Land Rover 2.25 and 6 cyl petrols, Commer 6 cyl, etc. by the hundreds.