Before the VRC-12 family there was an all vacuum tube family of FM tactical radios like the RT-67, RT-68 and smaller sets like the RT-70 along with their companion receivers like the R-108 If you watch the YouTube movie on the VRC-12 they show that family of radios that were the GRC-3,4 and 5
They were introduced in the fifties and were mostly removed from service buy the late sixties with many surviving in National Guard units until the seventies. Take a look at:
http://radionerds.com/index.php/AN~GRC-3And see if that looks familure. Maybe they kept them around for training that late?