I have balances that need replaced. The channel is 9/16" in width, the shoe has a 1" depth and the length is 25". The only marking I see on it is by the shoe and reads; "23 40 BSI MM". The spring looks like it's 13.5" with the cord fully retracted and not strung out. These are for my parents windows which are completely corroded. What would the pricing be per single unit, pair, or set of 4 units, or is there more savings per unit the more you purchase beyond that? Thank you