The piece that holds the spring has broken, and I'd like to replace the whole channel thing if I can identify it. It looks pretty much like your 795 series with the 1-inch wide bottom lock thing. But mine has a cord and some pulleys inside, and I wasn't sure if that's what your part has. The channel is 9/16 wide, and it's 36 inches long. The sash weighs about 22 pounds.
I see 36C and NK BSI on the channel itself, and a patent number 4610108 on the bottom lock piece. The pulley piece that broke has a 6 or 9 on it.
So would this be your
S795-36, option 36C?