Thanks for the responses.
In examining boxes of Remington Gun Club (12 gauge, 1 ounce loads) and Winchester Target (1 ounce and 1 1/8 loads) shells, I see some encouraging marks.
They all carry the CIP imprint. The Remington box even has a BNP mark with crown...and so I'm assuming they're considered safe to be fired in British guns within proof and with 2 3/4" chambers.
I'll take a look at B&P and Rio boxes when I find them.
Unless I'm misunderstanding something, it looks as if there are several American shells (besides RST) that qualify as lower-pressure loads.
