Thanks for posting! Like you said, this attachment combination doesn't match any of our existing Series. As such, your balance will have to be custom made.
Before I can begin, though, I just need to know the weight of your sash. I wasn't able to cross reference your stamp of 15 9 to anything, so knowing your sash weight will help us determine the appropriate option.
Can you also confirm the length of the metal channel? With a stamp like 15 9, I'd think it would be 16", not 17". Make sure not to include the nylon attachments in this measurement. Please double check and let me know!
The length of the metal channel is 16 inches as you guessed and the weight of the sash is 15-16 pounds.