What option should I pick?

A quick learner from jamestown says:
I have a window balance, and it is channel type. but doesn't look like what you have. Mine is a rod with a spring on the outside of the rod. special patent by Amsebury windows. The rod is 29 inches long. But I know what you have will work, I'm looking at the 795 series.
The question is what does the 29d, c, b, a mean? I think it might be the force of the spring, but not sure which I should use.
I have single hung Tilt windows, which measure 35X31 with balance on each side. Double pain. Not too heavy. made from aluminum.

Could you help me with which option I should order.

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Donna from SWISCO responded:
The options listed for SWISCO series 795 are the numbers stamped on the original channel balances for this particular window balance system. The options indicate spring strength.
To find out what option is needed remove the window sash and put it on a scale.
Stamp option 29A will carry a window sash weight range of 7 through 12 pounds.
29B, 12 through 18 pounds.
29C, 18 through 23 pounds.
29D, 23 through 28 pounds.

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.
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