Hello there. It looks like you might need an S795-21 Channel Balance, stamped 21E. It's tough to tell from your photos, but you might need the 15-195B T-Lock Shoe with the 'open cam', sold separately. It's easy enough to swap this out with the T-Lock Shoe that comes by default with this balance (the 15-195 'closed cam' shoe). See the guides below for an explanation. Also make sure your balance attaches to the top of the window track with a screw clip, rather than a hook. Let me know if the latter is the case.
Yes! You can pop our 26-200 screw plug in that hole to allow our Series 795 to screw in place. The video below will go into more information. Otherwise, I'm very confident that this hardware will work for you.
Anytime! Best of luck on this project and please reach out if you had any follow up questions. We're always here to help.