I have four of these shells from a trade years ago. I know nothing about them. I looked up info on Winchester Double X Magnum but couldn't find anything conclusive.Not sure what the 1B means. I don't shoot my shotguns very often.
I assume they are some kind of shot based on the end of the shell.
The "1B" simply designates that it contains number 1 buckshot. I don't know about the "magnum" designation since it is a 2 3/4" shell, but I'd guess it was loaded a little hotter than standard shells at the time. Nothing special about them. Shoot them, or cut one open if you want to see what's inside.
2 3/4 magnum shells are not loaded hotter than non-magnum 2 3/4 shells. The magnum designation means that it has a heavier payload of lead shot. The velocity is actually less than standard payloads because of safe chamber pressure limits.