Wile E. Coyote

Aah. So much for speed reading. It looks like the same weird power connector used for the GRC-106 judging by the pictures, as the female portion of the threaded center connector is on the equipment end for the GRC-106 and the MX-7777, where for the common VRC-12 stuff it's the reverse. A few Ebay vendors sell the GRC-106 power cables often enough if you want to gamble away...12 bucks or so. The polarity should be the same, but pop the MX-7777 covers and check to see what wire goes where first.

Speaking of 12 bucks, Mike Murphy has the MT-1898 for the R-442 receiver for 12 bucks here: U.S. ISSUE COMMUNICATIONS EQUIPMENT AND RELATED ITEMS (about 3/4 the way down the page.) They look very used, but you don't see them cheaper.