Hoyt,
RS and RO don't ring a bell right now.
Circle-S is found on the left receiver of some Police Lugers, usually in front of the serial number stamped there. The meaning is obscure. Many--in fact, probably most--collectors assert this to be a Simson rework mark, but there is no documentation to support this.
There are many other encircled marks, signifying other meanings.
--Dwight
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