There was a block from 1 400 000 onwards that was reserved for reconditioned motorcycles (C prefix). There were similar blocks for reconditioned cars (1 500 000 onwards, M prefix), trucks and lorries (1 600 000 and 1 700 000 onwards). But there were no numbers higher than 1 800 000 in this series. But for as far as I know, there were no blocks per make within these "rebuilt" census numbers.
The war time brass "Ministry of Supply" plates are smaller than the post war plates. Post war plates are also often in aluminium (1950's).
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.