19" sash channel balance; stamp #1740
Thanks for posting. Do both the top and bottom fittings lack wings, or 'side tabs'? See the diagram below of the bottom fitting as a reference.
If either of your fittings have side tabs, please send clearer photos of them. Otherwise, I believe you have either the S351-18 or S390-18. The only difference between these two channel balances is the depth of the bottom nylon shoe, or the 'heel-to-toe' measurement, so compare both with your old hardware to see what matches best. See the diagram below for reference. In either case, for a new stamp option choose 1740.
Is there an acceptable grease to use on the tracks?
These balancers keep breaking. They don't seem heavy duty enough to support the window.
Rather than use grease, we recommend a silicone spray applied directly to the window jamb track. It works just as well as grease or oil but without collecting as much dirt or debris.
As for the balances being too weak, that's easy enough to figure out. Before you buy anything, weigh your sash on a scale and get its exact weight. Then, review the chart you see on each balance's store page. That tells you exactly what kind of load each option supports. All you have to do is match your sash weight with that chart and pick accordingly.