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So it is (not sure why mine has a later date, looks identical apart from post dated annotations) still, the very end of ’77… I can’t imagine they rushed them out before new years eve

They did use old pots as you know which does seem the much more likely answer.

As for the #3, some of them had stuff scribbled on the boards, no idea what it means but that’s not the only unit I’ve seen with numbering.

Sometimes IC’s had dating, post a photo of the component side too.

Yes, quite – it’s 1977 but only just!

I had a (very careful) look at the chip side of the board. Haven’t got time to post the photos here but I wrote a list of the six chips:

RETICON SAD1024A 7328
S SCL4013BE 7713 24
RC4558NB RT7715
UA741TC F7649
S LM324N 7729
S LM311N 7741

I’m no expert on chips but I believe the four digit code is the date code, so we could have one 1973, one 1976 and four 1977s, the latest being October 1977.