Your bike left the factory on 2-10-41 and went to the Ministry of Supply. Your bike had C4555741 as tank number if my calculations are ok. Last time I made a mistake and Rob Miller proved me wrong, I hope Rob will see if I did it ok this time.
Here a photo of what it probably looked like when new.
I thought that it was one motorcycle so I only gave you the info on the frame number.
Your spare engine left the factory 2-4-42 and was also send to the Ministry of Supply as a complete motorcycle of course.
I can email you these pages with the info if you want, it gives quite a bit of work for me to make them suitable for uploading to photobucket while it's easy to email it.