Acorn Series 3500 Channel Balance

I already ordered two S351-29 29" channel balances with option 2830. But, this was unfortunately the wrong item. So, I need yor help. Our local window guy says he's never seen these types of balances where the "top" actually slides up and down in a groove on the window frame. I'll attach pictures that should show you everything I have. The length is correct at 29", but the "top" and "shoe" were wrong. My existing balance is stamped with 2830, BIS (sideways) and what looks like a small 1B also (hard to tell). The house was built in 1978. The "top" has grooves that slide up and down this channel in the side of the frame. The "shoe" just slides up and down on the outside of this channel. To place the "top" into the channel you unscrew a little rubber bumper near the top of the frame. Then, you can move the balance up to a spot where it can be pulled out of the channel. Hopefully, the pictures will tell you everything you need to know.
Please help.

Thank you,

John