Hi Maarten. Henk sent me measurements for the first one I made, but I used that Ariel O.C. spec for the second one.
At first I thought they were used for only the BSA 1940 model year, but they were definitely fitted to the KC10's during May-June 1939. They were also fitted to the 1940 model WC10, but these were also supplied in 1939 (Aug-Sept).
I think they carried on fitting them to M20's until stocks were gone, but carried on using up the number plate with the divot in the top which where also punched to take a standard MT110.
The earliest image that I know of with the MT1130 is C389369 from Contract A/9764 which was for 298 extended to 398 machines and commenced delivery 27/10/1938 with the last machines arriving 23/1/1939.
C389369 was the 225th serial number. Looking at daily arrivals at Chilwell, this would suggest November 1938 production.
Anyone have a photo of a KM20 with a lower number ? The MEE had KM20 318 (C389161) late in 1938. I'd be surprised if there wasn't a photo of it somewhere.