We've built some equipment with 75 ohm connectors. In those network
analyzers that use a separate S-parameter test set, you can often get
the test set in a 75 ohm version as well as the more common 50 ohm.
But the 75 ohm units are (as far as I know, _always_) labeled with a
caution about using the right impedance connectors.

Cheers,
Tom