Thank you all. I find the information about the dates of the 3 ton stamp on the barrel flats most interesting. I assumed it came with the original London proof somewhere around 1890. If it didn't exist then, it must have been added with the Birmingham nito proof.
I am confused about PSI and PSI by the piezoelectric system. These two expressions of pressure are substantially different.
Does the 3 ton stamp on my 1890 gun mean regular PSI or piezoelectric PSI. It seems to matter a great deal.

Richard Howard