It's what came on the bike. Granted it is odd to see such a thing on a bike ten years after most companies stopped using it, but it works. this stuff is the size of SIS shifter housing, lasts forever unless horribly kinked, and can be oiled from the outside to provide slick shifting. Spiral brake housing even filed flat, and the vinyl covering removed, will not fit iinto the collar on the dropout. It's an odd size no doubt. I guess the collar on the drop could be drilled to accept a ferrule, but I have no earthly idea if the material is thick enough.,,,,BD

Yep that's my bike. I snagged it for $70 + tax at a pawn shop. A 1985 600 model. I will take some proper pictures of it soon. It was converted to 7 speed early 90's Shimano 105, with mid 90's RSX STI's. My 86 520 has the same housing at the RD.