Interesting two questions. My view, while the gun is proofed for 1 1/4 both you and the gun will be much more comfortable shooting 1 oz or even 7/8 oz loads at 1200 fps. That load will break most every clay and kill most birds except wildfowl.
I doubt you are planning to use this old Purdey for your everyday sporting clays rounds so why not leave it as is and use it accordingly. I say this while at the same time I am using a 1937 J Dickson boclock for clays and it is choked .020 and .031 and would be better at .010 and .020
