Try a Google search for 204-15578-1. I got one hit. Also look for
68999 in the pdf file at
http://www.timesmicrowave.com/content/pdf/tl14/28.pdf, and do a Google
search on '68999 coax'. Things like that might get you somewhere...or
might be dead ends.

Also, measure the impedance. The size will give you a good indication
of the loss, or just measure it.

Cheers,
Tom