Tin grip Webley Junior pistol mainspring spec'

[Benelli B76]

Good afternoon,
I'm in the process of rebuilding a pre-war Junior and am now at the stage of looking for a new mainspring. Advice seems to be that later Junior springs will not fit the wooden and tin grip models (so say Protek) but the knibbs website identifies the same spring for mk's 1&2. So, I'm hoping for some guidance (or even pointing in the right direction of a source) on the correct spring.

Just buy one from troubledshooter.

Baz