The Truth on 75 ohm BNC…
“In spite of that, SDIF3 is 75 ohms. Not that I’ve ever minded. That aside, most equipment terminates at 75 ohms (if they terminate at all, big if in word clock connections) but the connectors are 50 ohms. This is because the pin of a male 50 ohm BNC is thicker than that of a 75ohm one. If you stick a 75 ohm cable part into a 50 ohm chassis part that usually works. OTOH if you do the reverse you have to jam it in after which the 75 ohm chassis part is damaged and will no longer work with 75 ohm cable parts. Because of that, most boxes have 50 ohm connectors installed. At least nothing will break. At frequencies encountered in word clock and SDIF2/3 connections, the impedance mismatch problem at the connector is academical.” [link]
H i F i D U I N O 
The comments are not entirely correct. A 50 Ohm and a 75 Ohm N connector are not intermateable without damaging each other but the original 75 Ohm BNC with a smaller pin really doesn’t exist anymore anywhere. The current solution uses the same pin and socket for 50 Ohm and 75 Ohm but the 75 Ohm replaced the teflon sleeve with air. It does hold 75 Ohms up to at least 2.5 GHz with a low SWR.