In spite of all the nice thing's y'all have said about me - I'm no big expert on the HW 30! But a gun from 1976 likely was indeed an ARH (Air Rifle Headquarters) import with a leather piston seal. If it has 13mm scope grooves, that's very likely one of the ARH special peeps.
Personally, I love leather piston seals. Just keep them lubed and they will last forever and be very consistent - I typically chrono with 15-shot strings, and the smallest standard deviation I ever recorded was with a leather-sealed Diana 27 of about the same age. Yes, you lose a few FPS compared to a plastic seal, but this is negligible on such a small powerplant. And I love those old 30's with the skinny finger-groove stock; the later R7's are nice of course, but to me the old stocks are a more natural fit for the compact, slender action, and better balanced.
Good chamber lubes are the old Beeman Chamber Oil, RWS Chamber Oil, Abbey SM 50, or stiff silicone oil (do NOT overdo the latter, you don't want silicone sloshing around in the action beyond the leather seal). The old Beeman Ultra Lube was re-packaged SM 50, by the way. Judging by the excellent condition of this gun, shooting a few drops of lube through the transfer port will likely do the job, but you can always take the piston out and give the seal a clean and soak in fresh lube if needed.
A more important thing to check on a gun that age is the breech seal. It might be leather, or crumbled plastic. If any doubt about how it's working, pop in a new one, they are cheap and it's an easy job.
As far as I know, the HW 30 and R7 have always used the 25mm piston seal, but I am not up on newer examples.