Constant force coil balance

I am unable to find any other markings or numbers on this balance other than the number 840 on the spring.
Would like to replace the entire mechanism. We bpught this older home about a year ago and can't identify the window manufacturer. Says Therm-O-Loc on the latch if that means anything. Any help would be greatly appreciated. George O'Leary

My best guess is that I should order 2 each of parts 15-140, 15-143, 15-144, 15-145 and S150-8 (window weight is 16 lbs) to replace both balances. Does that sound correct?
Thanks.