This is so weird...here's what the pattern says for 2 symbols:
\ = RS - knit 2 sts tog, WS - purl 2 sts tog
/ = RS - slip 1, k1, psso, WS - knit 2 sts tog purlwise
It looks like they're differentiating between k2tog, p2tog, and k2tog pwise.
Since the stitch in question is on the WS, it should be a left-slanting decrease, yes? Guess that would be plain ol' k2tog?
I've been bouncing around this question for a few days on a few lists, it's apparently a real poser!