Looking at what I can see of the board, it looks like this could be based on the Bailey three transistor design, which appeared in December 1966 WW. This had the innovation of feeding the frequency selective feedback from an emitter follower rather than the two transistor gain cell. (In addition, one of those transistors looks like a Ferranti E-line, now made by Diodes Inc. One of the secrets of low voltage noise designs are the E-line devices)
https://worldradiohistory.com/UK/Wir...ld-1966-12.pdf
It was further improved byWalker WW May 1972
https://worldradiohistory.com/UK/Wir...ld-1972-05.pdf
But that post-dates the OP's unit.
Craig