There are websites that specialize in older/vintage knitting patterns ... you could check out some of these sites and see if you recognize the pattern. I took a look at this site... there is a whole section on capes, ponchos and shawls (and several of the patterns are from McCalls).
You can also get vintage back-issues of McCall's Needlework & Crafts magazine from sites like this one... although you would have to remember which issue you got the pattern from :shrug:
Maybe someone on here has old issues of McCall's Needlework & Craft magazine that they could flip through (assuming you could narrow down the approximate year) ?
Just a few thoughts .......