Beware of many ebay sites that incorrectly sell and display 50 and 75-ohm connectors
(Physically the main differences can be found in the center pin and dielectric insulation. The 75-ohm BNC connectors feature Teflon as a dielectric and surround the outer spring fingers with air. The center pin maintains a consistent diameter in both the front and rear areas. The 50-ohm connectors use Delrin or Teflon (PTFE) to surround the spring fingers and the center pin is larger in the crimp area.)
See the last picture in the Preview Gallery.