Hi Nick,
I have a 1972 pre safety catch HW35. I have just rebuilt the action and sourced my shims from Bagnall and Kirkwood (do Google search) they fit as they should. The first one slides in easily, however, and I assume this is due to pressure from the spring in the barrel release lever, the second one took a bit of gentle persuasion, so I would advise fitting the shim on the lever side first.
Hope this helps.
Mike