I've no idea why BSA put the groove in Ian --- it could be something to do with how they're formed.
The 77/97 seal has the groove round the middle so that it can deform when compressed, the break barrel seal doesn't need this groove.
I find the break barrel seal works well in 77s and 97s if there is a bit of play in the cocking linkage and makes up for the wear.
All the best Mick