I have several FWB MK1's 124 & 127's, I'm not so sure as to your date of 1978 though, I have for example, a mint 127 serial No 25552 which was bought new in London in 1980 by BBS member abewilson from whom I bought the rifle from 2.5 years ago.
The rifle also clearly has a MK11 stock on it and I have never seen a mk1 with a wrap around foresight before ?
You mention 'Spanish Correos' So I can only assume it was imported from Spain, perhaps guns sent there originaly where configured diferently?
Not certain how someone could put a value on it of £ 300 when I only paid £ 260 for a Genuine 'Mint' mk1.
Hope this helps rather than confuses