Serial numbers

[monopix]

I'm looking to date a 139 with serial number 124***. Has anyone got any information that might help? I'm thinking maybe anyone who has one with similar serial number that knows when it was bought new.

I have one with serial number 055*** which I bought new in 1981 so I think the other must be mid '80s perhaps.

Also, when were 139s last made - that would give me a latest possible date.

Thanks.