The stamping of the WD serial number doesn't pre-date the use of contract plates but seems to have been a pre-war practice dating from the time when there was little or no attempt to tie serial numbers consecutively with frame / engine numbers.
It's common to see this on early WD/Cs as well, and always with the large font 'C' which suggests that it was done not in the factories but at RASC depots upon receipt, probably at the same time as they applied the painted number.
It's a wonderful thing to have of course and it has helped Jan to show that the RASC serial numbers were applied in a rather random way to the machines early in the war, whereas the RASC seem to have tried to apply in consecutive order from September 1939 onwards and to have discontinued the habit of stamping the numbers on the cases (they didn't use the 'C' by the way).
There are surviving brass contract plates going back to 1937 at least.
Whoops. I really wasn't paying attention last night ! I thought that it was the old 3SW habit of stamping serial numbers on the cases.
This was a slightly odd 3HW contarct as it's marked "2129 RAOC. 3366 RASC."
I still suspect an RASC involvement in this...It is possible if the contract was extended, that there were not enough pre-etched contract plates. As far as I can tell, Norton contract C5109 had no plates (it was extended from 2000 to 5000 machines) and some of the following contract had cases stamped 294/C/5612 - but by no means all of them...
