21252hq,
Many thanks for taking the time to explain the proof markings!
I took a look at it this morning and decided to pass as the bore was a little too frosted for me but what an interesting rifle it is. It is engraved with Charles A Heyer and co. Nairobi. He was an outfitter in the early 1900s in Nairobi.From there it made its way to England in the 1970s and to Ohio in the States in 2019! I am pretty sure it was shipped just as it equipped now with the express sights. The 1939 Shooters Bible shows a MS high velocity rifle with express sights so they were available as a factory item.
As to the caliber, I took a resized 9x56 ms brass with me and it would feed about 2/3 of the way. Could be a tight chamber, which I understand is not unusual or something else. I would guess it is a 9x56 but a chamber cast is in order to be sure. I did not ask them to do one as I had decided not to buy it.
Again thanks for the informative responses.
My best,
Mike