After planting the seed by Jan on contract C3139, I have been looking at known census and frame numbers of pre 1940 WD16H contracts.
Contrary to what we assummed until now, it seems possible to calculate pre-war census numbers based on the known ranges of numbers reserved by Chilwell to the known ranges of Norton frame numbers.
Although there will always be a small margin of uncertainty as we do not know what mistakes the military made it is fairly certain the census numbers can be calculated in sequence with the frame numbers for most contracts.
Some contracts extended after the initial contract give problems which need further investigation.
C9681 is an example of that. I can match numbers for the initial 1689 machines but its muddled after that with the end of the 1937 block of census numbers. They started with 1938 census numbers after that. Original contract mentions 1786 which was changed to 1790 machines but amended further with additional 350 and 80 machines plus 2 machines for sidecar work with modified gear ratio's.
This gives a total of 2202 machines according the contract ledgers. There were in total 2259 census numbers reserved!
Keycards and existing engines with census number on them enabled the check on the contracts.
Finally a useful result from the Keycards. Out of 90 entries for this contract C9681, only 5 were useful showing frame and census number in the correct ranges. The rest were discarded for showing rebuilt numbers or incorrect frame numbers.
3 matches were found on engine crankcases for which the frame number could be found.
I think that it should be correct to use the same calculating system on the Big4's but I have no proof for that. They are not in the keycards but the system will most likely have been copied.
