Please don't rotate the barrel. I cant tell you the number of times that I've seen beautiful old Remington 51's buggered by trying to disassemble them like a Colt or FN.
I'm just sniping the disassemble part
Disassembly is actually pretty complicated. I suggest you purchase one of the reproduction factory owners manuals.
It's an inspection mark. If your interested in more information about the Model 51 there is a survey currently being conducted for them on the Remington Society of America under current research.
Model 51 manual located here - http://stevespages.com/pdf/remington_51.pdf
Model 52? ??? Model 51?
What Model 51 pistol or a rifle and what model?
Start with these guys... http://remingtonsociety.com/rsa/research/Model51