BU508A, you made a good deal!
TEA time:
I've purchased a Fluke PM6681 with the UCXO and no 3GHz prescaler for around 325.- Euro:
The Signal Path did a repair video on a PM6680B and a follow-up video about a OCXO module for the PM66xx series,
which he got from one of his viewers.
This is the website:
https://syncchannel.blogspot.com/2016/04/flukephilips-pm66xx-frequency-counter.html
I think, I'll assemble this module.
I have two of them and they are really universal in their function set and are spec'ed like the HP 53132A and better in some regards.
The user interface is much more handy with more dedicated buttons and less sub-sub-menu magic.
That's why they are my go-to's on everyday use.
Ok, the display is "only" 10bits but you can use the math function to subtract the expected value to get only the difference as a trick to extend the 10 bits.
The PM6681 has an internal 100MHz clock to improve the resolution over the PM6680 and PM668B by a factor of 10.
I've also seen shahriar's video.
-But I'm wondering why somebody with a lab would be going for OXCO's and not for a GPSDO and a lab-wide reference distribution?
There a very nice but low priced GPSDOs out there, the best one I currently own is a UCCM-LPK-module from samsung with ublox gps receiver; It works better with my poor sight of the southern hemisphere than my Trimble Thunderbird.
One question left: What's that grey plasticky looking part covering the bay for the channel C option?