The 'Rebuild' serials were presumably issued in blocks to the various workshops but it's difficult to detect a pattern. I have photos on file of the following :-
There are numerous 'Rebuilt' serials listed in the post-war Key Cards but it would be a major job to classify them. I have extracted the information for one particular Norton contract and the numbers quoted run from C1401546 to C1467636.
Have you asked Lex Schmidt if your frame appears in the 'Key Cards' ? He may in any case be able to give you an idea of the range applying to Matchlesses.