The fin sticking up is the lug which used to fix the pin fitting rear sight and this was used on the Mk3 Meteor. The Mk4 Meteor had a screw fitting sight base, you will find two holes in the barrel for this one larger with the smaller one tapped for the screw.
Both of these sight bases are plastic and break easily.
The Meteor was sold as a junior gun but that does not stop it being a good little gun if you accept it's limitations, It's reasonably accurate out to 30 yards, light and easy to cock.
They suffer from the buffer washer/breech turning to cheese as do all 70s BSAs.
Mk3s or 4s have fairly hard triggers but IMHO the Mk5 was worse.
Welsh Willy makes a drop in kit and piston seal these days.
There are alot on here who are fond of their Meteors myself included.