Great photos Curl, and perfectly done.

There is another fairly reliable way to tell - the four pins that make a straight line behind the cocking indicator on the back-locks.

Did you notice that the .577 is a "flat back" (not stocked to the fences) with a through lump? Both are features that "best" guns aren't supposed to have, but it says "Royal" on it anyway! I handled it in April at the NRA show. Nice rifle but the Cordite burn is significant.
-----------------------------------------------------