I doubt if it would be Bruno
The MK1 HW70 was introduced in the early 1970s, the 25mm HW77 seal wasn't produced until 1983.
So as you say the 77 seal will fit the later ones but not the pre 1983 guns.
Edit: after a quick search here's the earlier seal arrangement :-
http://www.muzzle.de/N3/Druckluft/We...w70_spreng.JPG
An adaptor and a Diana/Original 5G seal may be the way to go if the proper seal can't be found.
All the best Mick