A quick Google search on "120v transformer 560-0077" yielded several hits from the Gates Harris company, which apparently was, and maybe still is, a big player in broadcast radio, among other business lines.
I looked in a .pdf of a relatively modern manual, c. 1991, and found a photo of a transformer that looks a lot like yours.
Can't seem to paste the link with this iPad, but if you're really curious, PM me and I'll send it over.
-- Matthew