MI 4300 series double spring balance
Hello there. See the 15-252 Inverted Dual Support. This setup works with Series 160 Coil Balance springs. If the window sash these balances support weigh exactly 21.6 pounds, then use the S160-05 5.5 pound springs. Use (2) of these on each side of the sash. That will come out to 11 pounds of support on either side, for a total of 22 pounds of support overall--this is close enough to your sash's weight to work just fine. Let me know if you have further questions.
These unfortunately aren't sold as pre-assembled, complete units. We can only supply the constituent parts separately.
As for how they mount in the carrier, they're just cradled at each peg, like you show in your second photo. The springs hook into each other, like a barrel of monkeys, and then reach up to the top of the track to hook into the bracket there.
That's great to hear! Glad everything is functional.
Please don't hesitate to reach out if you had any further questions for us. We're always here to help.