Joe,
Yes, that little item does belong with the 90711 and, by the way, I assume you know how rare those 90711's are! Congratulations.
The normal 90711 output is low-Z; the little device is a "high impedance adapter" in case you're driving something that requires hi-Z. You plug the adapter into the 90711 output and there are various plug configurations on the bottom that plug into the crystal socket of whichever transmitter you are using.
There you have it! :-)
Here is my Millen station:
http://www.isquare.com/personal_pages/millenstation.htmGreat old stuff!
73, Bob
WØYVA
http://www.isquare.com/personal_pages/ras-hardware.htm