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
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 .......