According to Drags listings RBH was also used again in 1951, which tallies with the frame number. The OG timing cover is probably a replacement at some time and the rest is possibly all genuine '51, but as Bob has said, what a challenge!
I think the reg number may be HEW 958, (none of the other letters really matches the two vertical lines of the partially hidden first character) and this would make it registered in Huntingdonshire between Jan 1950 and April 1951 so supports the WA frame number
I notice that the vendor has taken the original listing down and has re-posted it as being '1950s' - which was the honest thing to do. Mind you, he obviously doesn't realise it has a pre-war speedo fitted which is probably worth more than the rest put together!