Chris,
Thanks very much for the link. It has answered most of my questions. Apparently 7-1/2 primers were introduced in 1965, "Specially designed for .221 Rem. Fireball, .222 Rem. Magnum and .223 Rem. Cartridges."
In 1978 it was specifically mentioned for use in the then-new .22 BR Remington, as it was "designed to withstand higher pressure loads." It first listed as the 7-1/2 "Small Rifle Benchrest" primer in 1980.
So far I haven't found any mention of it being listed as a "magnum" primer, but the Large Rifle 9-1/2M showed up in 1968 "For use in Belted Magnum Cartridges, .264 Win., 6.5mm Rem. Magnum, 7mm Rem. Magnum, .300 Win. Magnum, .350 Rem, Magnum, .375 Win. Magnum, .458 Win. Magnum." (I found it interesting that Remington didn't include the .338 Win. Magnum--and listed the .375 H&H as the ".375 Win. Magnum.")
Am still going to run a shooting test to compare velocity results with other primers, especially the CCI 450 Magnum--the present version which has a hotter priming compound.
John
