What option for my s751 -30

A quick learner from US says:
Stamp on my balance says "30" 1982 Marvin Window. I know I want the S751-30 but I don't know which stamp to choose. The sash weight is approximately 21lb.

It looks identical to ribbed s175, and the knife lock shoe also looks identical to, and has same dimensions as 15-014.

I need the "option" to order the balance.

Based on the silver insert on the glass "DS 82" the windows are probably wooden double hung Marvin (has name on locking hardware).

Ordering 26-151 and 26-152 as well as 19-029 and 19-030 s clips.
User submitted a photo of a window balance.
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Tom from SWISCO responded:
Sure, I'd be happy to tell you what option you need. Check out our 29R option. Two balances with this option can carry a sash weight range of 15 through 22 pounds. Your sash appears to fit comfortably within that range.
A quick learner says:
I reweighed the window, since I was not so sure the estimated 21 lbs was comfortably in a range that tops out at 22 lbs!
As it turns out, the window is really very close to 22 lbs.

So, wouldn't I be better off at the low end of the next range up ( more rugged than I need) rather than at the high end of a range that is barely rugged enough?

What are all the weight ranges for your options for s175-30 and -24, and -20, and -16.

I have all these windows with same channel balance hardware, but window weights and sizes vary.

Thanks for your help. I have lots of windows to repair.
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Tom from SWISCO responded:
If your window sash is on the high end of 22 pounds, then the next range up would be better. That would be our 29HD option. Two balances with this option can carry 22 through 28 pounds.

I think for the S175-30, that covers all the options except for 29TG. This can carry 28 through 35 pounds.

The other options you've requested are as follows:

23HD: 18 through 22 pounds
23R: 10 through 18 pounds
23TG: 22 through 30 pounds

19HD: 17 through 22 pounds
19R: 10 through 17 pounds
19TG: 22 through 24 pounds

15HD: 15 through 22 pounds
15R: 10 through 15 pounds
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