Mark,
Glenn and Kamilaroi are right! Once you get the book, you will be a real Farq. fan!
Yours is what is called in the book, a 3B action.
I have one very similar to yours which was originally a 450/400.
I would be interested to know if your action has a number stamped on the front. Say, a 450 or a 500.
If it does, that denotes the caliber size which was thought to be the max for the action and when placed next to an earlier action like a Jeffery, you can better appreciate the "bigness" of the 3B.
I would suggest a quick disassembly and check. Many of these guns have had repairs and replacement parts of questionable quality over the decades, but I must say, your gun looks to be fairly unmessed with!
Once you get the book, and grind some proper screwdrivers, you will see how service friendly the PD Farquharson really is!
Congrats!