> I thought the 639/640 were repackaged SCR-522s The BC-639 is pretty much a repackaged BC-624 RX w/o quad channel-changer rack. And the seldom seen BC-638 freq. meter has same front-end as both But the BC-640 is sure it's own animal w. p-p 24Gs in final modulated by 811s. As the man that donated a set in 6' green rack to Warner Robbins museum 10 yrs. ago