I believe your trailer would have been converted to the Coventry Climax engined generator in late 40's/early 50's period, at that time (or during a further re-build) it would have had the later electrical wiring and fittings installed.
BUT, think the basic trailer and chassis is WWII manufacture.
Taskers had contract S9778 to build "Trailer, 10cwt 2wh Generator 6Kw". This contract was allocated the following 'X-numbers": 6263828 - 6264327, although I don't know how many generators were actually built of the 499 numbers allocated.
Regarding the Electrical Repair cover: my trailer is currently in storage and not easily measured. If you are going to have the cover made, you could make the bows to suit and then the cover to fit. The cover is held on with short leather straps that secure onto footman loops which are under the body.