this is another photo from the top...

Without knowing the original maker.....

C.R. Laurence is a Huge resource for those fittings......Peruse the website to locate the likely (replacement) product and hit the "locate dealer" link and email them for a local or online dealer......

Here's the "sliding hardware" pages/link

http://www.crlaurence.com/ProductPages/showLine.asp?GroupID=35150&Path=39326::28293::28298::35150

"......measure Once.....cut Twice....

throw that one away and cut a new one...."