This is a blast from the past. I have been a service engineer on welding equipment for 32 years and I started with the UK Oerlikon agent at that time.
I worked on many Minifil Spool on Guns at that time. I am suprised there are any left working although they were of excellent build quality.
The gun was manufactured is Swiss, from a now closed factory. It was obsolete before Air Liquide took over Oerlikon Welding across Europe.
A large number of guns were sold to the MOD. The Head Socket was the main problem as I recall, 2 types existed.
If I can be of help please contact me.