Hello Ian.I have one like you have described.Mine is dated on the brass screw on plug as 64 3 .If you look inside a chrono speedo from the sixties,even the brass chassis is stamped SMITHS MOTOR ACCESSORIES LTD.ENGLAND,somethig you dont find on 40's or 50's production.So I would say It's late production as they probably kept making them as long as there was a demand.Regards Sam.

email (option): smarwick@iprimus.com.au

Thanks Sam..as I suspected a later replacement. I guess they had to meet the demand from the services until the M20 went out of service in the late 60s...Ian

email (option): ian@wright52.plus.com

they were still in service in early 1973 in cyprus when i left the unit

email (option): roger.beck@node6.com