I was wondering where I got the idea about the limit and after a bit googling I think I found the source. It's an old usenet posting by Dave Eadsforth from 1999 and I don't know how accurate it is:
"One small amendment concerning the Tempest, it carried 150 rounds per
gun. I believe that almost all British fighters were ultimately limited
to 150 RPG because the Molins Belt Feed Mechanism, with its wind-up
spring, only lasted for 150 ish rounds.
Martin-Baker produced a 'Flat Feed' mechanism later in the war which
could feed unlimited rounds, but Sir James was pretty unlucky at getting
orders for his own kit - until ejection seats, of course."
The Molins mechanism should be about same as the french Chattelerault mechanism. I wrote also to Tony Williams but no reply so far. Anyway, my value for the limit was wrong.
gripen