I had a 1978 .22 HW35 Export, a very nice example. Power was under 10 fpe with a new standard spring (leather seal). On closer inspection there was a tiny lip of metal on the inside end of the transfer port, looked like the drill bit or whatever tool they used had not quite gone all the way through. So I found a metal drill bit of the right diameter and gently twisted it through the port and cleared the lip. I think it was occluding about 10 per cent of the port at one end. Result? Without any other changes this liberated 1.5 fpe. So make sure your transfer port is open at both ends - COMPLETELY OPEN!