[Arran]

  I thought that the 6A7 or 6A8G may have been in a location like that, at least the socket is not too noticeable from the back. Depending on the quality of the installation it may or may not be worth changing back, it's something you could do whilst the pushbutton bank and IF cans are rewired. Some servicemen did decent work, others liked to cut things out, tack things together, or piggy back old components with new ones.
Regards
Arran