Single hung window, hard to open, right balance sticks in 'up' position

I just acquired a house built in the late 90's, and have a window on the front of the house (single hung) with a pair of spring balances. The window is very difficult to open, and when I do get it open, the right balance sticks in the 'up' or open position. While they don't look broken, I'd like to find the right replacement parts to install two balances on that window.

The markings on them are "27 3 HN CALDWELL". The channel containing the spring is 1/2" wide. They are 29" end to end, and 9/16ths inches wide and deep.

Please help me determine which is the right part to order.

Thanks!
Gordon

Thanks Paul. It's odd that the web site link in the home page menu titled "Identify your Part" doesn't take me to the link you provided, it's very helpful!

Anyway, an update: the channel itself is 28" long. I've attached photos of the ends. I'm thinking a pair of Series 380, 19-005 Top, 15-005 Shoe? Also, it's not clear to me what Sash weight range to select? I do know the window is pretty hard to open now, so perhaps it needs a stronger balance? The sash weighs approximately 24 lbs.

Thanks again
Gordon