It appears that I need a Series 751 balancer. The stamping on the current balancer is "BSI 26HB JK". I'm curious what the HB means? I will take a closer look when I get home this evening.
Tom from SWISCO responded:
October 13, 2016
I've never seen "HB" before for this kind of balance. I recommend weighing your sash on a scale and referring to the chart we have on the S751-27 store page.