The W&S Mk2 will always give poor consistent accuracy when using the diopter sight because the bolt will not return the breech end of the barrel to the same point every time. The harder the seal the better to combat this fault but over time that will cause the sights to wander off.
Bowkett had a customer many years ago who worked for Greener but on the toolmaking side not a gunsmith. While working there he wanted an airgun so got them to make one up for him. Apparently if a dealer wanted a Greener airgun they put one together from components in store to add to an order for rifles, shotguns or whatever. After the launch there was no production line as such. Bowkett did a deal for the put together Greener and sold it to his friend the late Ray Hill. I believe Ray passed it on to a very skilled chap. When the ex Greener toolmaker had it it was in the white with no markings or numbers. The skilled chap rectified that and blacked it.