It would be hard to imagine a better gift for a radiohead, especially with your affinity for Command and ARC-5 sets, Mike. The RU set is a precursor of sorts to the ARC-5 types as I recall, actually coming from the BC- Army equivalent. Mike Hanz can probably rattle off the details and dates without missing a beat, hopefully he'll spy this thread.
The -18 was a stand alone receiver with no matching transmitter, 24-28 volts IIRC. The -16 and earlier were 12 volts for the most part, and the matching transmitter was the GF-11, also black wrinkle. You could pick up a GF-11 pretty easily if you wanted to match it up better, but I bet you're quite happy with it as-is for the personal history alone.
One of the neat things about the RU type receivers is the ability to remove the top tube cover and then attach one of the ARR-1/ZB-3 beacon set to the top. There's a pic of one atop a nice RU-19 near the bottom of this link:
http://home.versatel.nl/gmwzijlstra-prummel/SCR74.htmlAbout 10 years ago I had a friend in Colorado gift to me a nearly complete NOS RU-16/GF-11 set up with all the coils, control boxes, mounts and dynamotor. Was missing one connector, the spline cables, spares kit and manual, all but the splines and one spline switch have been located. I hope to have it mounted on some sort of transportable surface and bring it to NEAR-Fest one year.
Maybe you'll bring this one to operate sometime? Nothing is quite as cool as a set run by a dynamotor!