Hi guys, sorry for the delay, and thanks for the replys! I solved my own problem when I went to look at the model number. It's a #170, 1/4". It has a grip switch on one side, and a similar grip plunge lever on the other handle. I was looking at the model plate, when I noticed that one of the screws that holds the case together was located near the plunge lever, and the other side had no matching screw hole. "hmmmmm" I thought. Sure enough, that's the adjuster. It looks just like a case screw! That was the problem too - once I gave that a quarter turn, it locks up tight now.
Hopefully someone else can use this info someday.