Windows are side tilt, aluminum frame. Channel is 1/2" x 30" measured. Only markings are 3050. Which slide assembly is needed? It appears that the 30" Series 351 is a 2950, while the 31" Series 351 has a 3050. But my channel is 30" long.
Which assembly number is applicable?
I believe I know what replacement part you need, but can I see a photo of the bottom shoe on your balance. Since the one you photographed is damaged, it's hard to tell what this will match up to. Is there an undamaged balance that you could take a photo of? You could also measure the depth of the bottom shoe and that would be helpful as well.
To start, it looks like these are Series 385 channel balances, not Series 351. Series 385 has a 1-3/32" shoe, shown below.
The only confusing part is length. If these are in the same window, they really should be the same length and have the same stamp. Measure the metal channel for each balance very carefully to confirm that they're 30". 2950 and 3050 would usually be S385-30 and S385-31, respectively, but you say both of yours are 30". I highly recommend double checking just to be sure which of these would work best.
Once you choose a length, you'll need the weight of the window sash that the balance is holding up. Take your weight measurements and compare them to the chart on the balance webpages to see what "weight option" you need to select when you order. Let me know if you need any more help.
Continuing:
Both windows are same size and weight, 21.2 pounds. Pictures show one channel balance from each window. Each is same size, but in image with tape measure, see that one is marked as 29-5 AA CMC 11, the other is marked as 3050, nothing more.
The shoe is 1 3/32". Channel inside spread is 1/2".
Stumped?
Thanks for clarifying! If both balances are 30" long, then it's possible that whoever installed these originally simply cut a 31" long balance down to 30". It happens all the time, and it'd explain why a 30" long balance has a 3050 stamp. You're probably safe going with the S385-30 channel balance in both cases.
Speaking of the stamps, yours don't seem to match your sash weight. For a sash weighing 21.2 pounds, I think our 2930 option would be better than 2950. The 2930 can carry sashes weighing 16 through 23 pounds, but the 2950 carries 28 through 32 pounds, which would be too strong.
