I've come across this notation in some logic notes regarding Henkin's proof of completeness:
$W_{LC} = \Upsilon_{i=0}^{\infty} W_{LC(i)}$
I believe the character is an upsilon, although I suppose it could be an uppercase Y. It's not explained at all, I've never seen it before, and evidently no one else on earth has either because I can't find anything about it online. Anyone have any thoughts?