We're a small parts testing
house)
I assume you do testing for profit, this is an Amateur Radio newsgroup. But,
since you asked :-).

These are definetly feedthru caps and they want
them verified to be Zo of 50 +/- 10.

The Zc of a capacitor (capacitive reactance) is frequency dependent. Not sure
what Zo of a cap. refers to. At some frequency the capacitor will have a
Zc=1/2*pi *f*c. I am sure you know that. An ideal capacitor will have a
Zc=0-jZc. A real world capacitor will have a non-zero real part of R+-jX. To
have the real part 50 ohms( Zo), must mean the frequency is in the VHF region
or higher. To measure what you want, you need a Z meter with the capability of
measuring the complex impedance of the feed-thru capacitors at the frequency of
interest. I have an antenna analyzer that I use for that sort of thing, but it
only reads capacitive reactance at the frequency of interest.
Not sure if this helps?
73 Gary N4AST