I might be able to shed some light on the crosshead guide or at least the material origin.
I think the R is a P with a scratch giving appearance of an R.
In the Boston area, the Chase Parker Warehouse or Chase Parker Co.
was an industrial and machinist supply company run by the Chase family who were my neighbors where I grew up. It is quite possible that the mark was there as the brass stock was delivered from the warehouse. They shipped all over the country from the Boston area warehouse. When my boss during my school years needed supplies, he would phone in the order and one of the owners would bring the goods home with him and drop them off. I don't know when they were founded but I know they were in business back in 1905 and they survived into the 1980s.
Edit: CORRECTION:
See Nick's update below.