Hi Louis,
That looks like a nice example of a c.1929 Standard No 1 with the so called 'Transitional' stock.
I'm not particularly technically minded, so can't comment on rifling but it generally does not wear out on old BSAs. It will last several lifetimes if lead pellets are used rather than nails, BBs or other inappropriate ammunition, albeit a lot of misuse would be needed to wear out a BSA!
The velocities you measured sound about right to me; brand new, these Standards shot at around 600 fps.
The piston washer looks a little worn but if the rifle fires smoothly, I would be inclined to treat it with oil, as you have done and leave it in place. Protek should be able to supply a replacement that fits if you need one.
Kind regards,
John
Currently looking for Baikal Makarov pistols with the following prefixes to the serial number: 98, T01, T09, T21, T22
Prefer boxed or cased but will consider loose examples too.