@ Coax Gang : thanks chaps for helping clearing everything up !
Oh not this again.
The ORIGINAL 75R BNC had smaller center contacts. It was constant impedance. The problem was it is not intermatable with 50R. Putting a 50R male on a original 75R female damages the female contact so it won't then make proper contact with 75R males. A original 75R male won't make proper contact with 50R female.
This issue was "fixed" by using the 50R center contacts with cut back insulation. This is inter-matable with 50R. However even two mated new 75R BNCs are not constant impedance. At video and other low frequencies where 75R is mostly used the short length of the discontinuity has little effec on performance.
They also make 75R N types (and C) but these always use thin ceneter contacts. If you come across a N connector with red insulation or painted band inspect it carefully, it may be 75R. I have a R&S 75R SWR bridge that someone ruined by someone (not me) screwing a 50R N onto it.
Thanks, that's clear. So if I understand what you said here and in your other post, then the original 75R plug had a thin white dielectric piece, and it got removed altogether later to achieve the 50R mechanical compatibility, since one has to maintain the OD/IN ratio constant for a given impedance. So since the OD of the inner conductor was made bigger to match the 50R one, so they had to make the ID of the outside conductor larger as well, to keep the ratio constant. So they removed the little white dielectric that there was, completely, because that's the only way they could increase the diameter.
Now that I actually know the reason for everything, it all makes sense and will be therefore easy to remember
So that's cool then, 90% of the stuff I have in that box of random bits, is indeed 50R, Radiall, I am am so happy, excellent score that was