Just to expand on this: The Campagnolo housing clamps show up a lot. There were just some Shimano on ebay. Also made: Universal, Simplex, GB; there must be others. The other Italian companies like Galli, Gipiemme, Ofmega, Ballila? Did Mafac and Huret make housing clamps? Can't forget Weinmann either. A seemingly simple item you don't see much of. I'm sure rust got many of the old ones given their location on the top tube.
Pete Geurds
Douglassville, PA