Channel Balance and Top Cap

I need to replace the channel balances on a couple of my windows. The model number of the window is Mark 40/50 STD Fin (pic included), the channel balance is 30 inches long and has a marking of 29 4 BS1 or BSI and it looks like 11 after it. I also need a top cap and it appears to be a plastic piece at the top that hides the top of the channel. I have included pictures of all. Thanks for your assistance.

Here are the ends that are riveted on to the channel.

That looks like it Thanks.