OK I have re-examined the 42-year-old faint carbon copy of the ticket.
It definitely says BE576 Heathrow to Geneva. BE577 Geneva to Heathrow.
However underneath the typed BE577 there is a faint handwritten "BE625".
I have also found the boarding pass - BE625 Geneva to Heathrow 1/4/78.
So, which type would it be, another Trident? And why the change?
Many thanx.
Goldox.