Channel Balance, plastic bottom seal, pile weatherstrip and Sash tops

Hey, Steve. As long as your nylon fittings match the 19-002 and 15-002 that come with the Series 351 then it should work well. That said, it is not normal for two balances in the same window to have different weight ratings, no. Somebody must have made a mistake during installation. Based on your sash weight, you should go with 2120 for both.

I don't recognize your weatherstripping from this photo, unfortunately. Can you show me a clearer picture of the end profile?

Finally, regarding the top sash guide, our 18-166 is very similar but you should go over our dimensions to ensure that they match up.

Great, I'm glad to hear our balance will work. 

As for this bubble seal, we don't have an exact match for this but it looks similar to our 58-171. I recommend just double checking your dimensions and reviewing the measurements we show on its store page to confirm whether or not it will work.