+ or - to Monster's Level would indicate an older printing of the set in question. As I have older sets of The Good, The Bad and The Munchkin and Munchkin Fu 2, I have some cards that read the old way.
If your cards say that, I'd recommend seeing if it's an older printing. The following quote is from the General Card Changes section of the Munchkin Change Log for the 2010 update found
here.
"All Monster Enhancers have had "Level of" deleted from their modifier description. Negative Monster Enhancers no longer say "Minimum Level 1," either."
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Andy Partridge
MiB #3282, Palm Bay, FL
Munchkin, Zombie/Cthulhu Dice, Burn in Hell!, Revolution!, Castellan, Chez Geek, OGRE DE, Chupacabra, Nanuk
mib3282@gmail.com