Stuart,
In the 70's I would think he was issued with one of these, it could be any one of the Fab 4 issued at the time.. They are not cheap these days and go for a bit of a premium, 6BB on the rear tells they were made for the RAF alone. see here
https://www.ebay.co.uk/itm/305297124...Bk9SR_itw_i0Yw
https://www.ebay.co.uk/itm/325982893...Bk9SR6Lw3fi0Yw
https://www.ebay.co.uk/itm/166018891...Bk9SR-j0jfm0Yw
https://wornandwound.com/time-spec-1...-chronographs/
You could of course go for a limited edition release of one of the watches with a quartz movement.
https://www.ebay.co.uk/itm/315150665...Bk9SR-j0jfm0Yw