[Maarten]

A Mitcham design seems plausible, but can easily be confirmed by looking at an actual set and its documentation.

Code numbers of significant parts depend on the place where a set was designed, so one would expect to see code numbers starting with MK instead of A3 for more than a few components in the documentation and in the actual set as far as code numbers can be found there.

Also, the place where a set was manufactured depends loosely (more or less so depending on the era and the kind of equipment) on where it was designed. If the serial number starts with M for manufacture in Mitcham rather than E, chances are it was at least partially designed in Mitcham as well.

Last but not least the presence of an execution/destination suffix behind the type number. This doesn't say anything about where a set was designed, but it can give an indirect clue. If a suffix /15 is present, the set was intended for the UK but might have existed in versions for other countries as well. The chance it was designed in one of the destination countries instead of centrally, is a bit lower in that case.