I have one of these bought from Protek in 2007 ser no NG43999 .
It is a standard meteor Mk IV from 1973 approximately but is fitted with a military style adjustable peep sight mounted above the grip at the rear of the air chamber. It seems they were called "Cadet Meteor", and intended for cadet training and available in .177 cal only and prefixed NG.
The Mk IV meteor in .22 was prefixed TG but this was NOT a "Cadet" and was of conventional Meteor style with normal adjustable open sights mounted in the usual place above the breech.
Hope this helps.
Aubrey