A short while ago somebody asked this and nobody knows.
I checked my small Meteor collection plus my Mercury S and got this very limited result. Just by loading superdomes into the breech.
All Meteor Mk 1-4 allow the pellet to drop in 3-4 mm without resistance, ditto the Merc.S.
My Mk6 needs the pellet to be pushed firmly into the breech just as my Hw99 does suggesting it is meant that way and I suspect it to be a genuine 5.5.
Sadly my Mk5 is .177 so can any one else do the same test then assuming we can date the various Mks, we might be able to infer a rough date.
The Mk6 barrel also differs in that it has only one hole for the front sight, matching the modern front sights.