With thanks to Dale for these pics. He says:
"Leather piston washer and large transfer port. Originally had a leather breech seal, but it was well knackered and replaced with modern breech seal.
Rearsight show different detailing to later HW sights.
Barrel has chisel style locking detents not the ball bearing and pin of later HW30’s, detent retained by a screw in the side of the breech block, not roll pin as on current HW break barrels.
13mm dovetail rail on cylinder with two arrestor holes, clip on rear cap on end of action.
Rekord trigger fitted as standard, early type with three piece laminate main sear and slot head adjuster screw for sear engagement.
Barrel is stamped: STAMPFLI WAFFEN LUZERN just ahead of breech block and the butt has a damaged Stampfli Waffen medallion inlet into right hand side."
[Dale discovered this is a surviving "gunshop based in Lucerne , Switzerland and are still in business, see website here: http://www.stampfli-waffen.ch /"]. He contacted them and they said: "They do not have records for airguns back that far, but... did say that with the stamping of their name in the barrel was done up to the late 1970’s, after that they engraved their name instead."