Hopefully this will help but cannot guarantee:
From my notes: (note that I was not aiming for high power)
' Original spring spec is, I believe c. 2.75mm wire x 32 coils. The replacement I put in is HW77 spec but seems OK. A meteor spring does not fit as the spring guide does not fit.
Subsequently fitted a 7.5J spring from Versandhaus Schneider'
Cheers, Phil