6E7 valve wanted Canterbury $20 - $40

A quick look suggests the 6D6 may a simple exchange - at least the base, pinouts and heater are the same, gain, bias and loading similar! Cheers, David

Thanks everyone for your replies. As Macbeth said to the witches regarding his suspicions of Banquo  "Thou hast harped my fears aright ".The 6E7  is a 7 pin valve and  fits a 7 pin socket.The Riders schematic showed a lineup of 6E7 6A7 6E7 6C7 42 6Y5

 As found the set had  what were  stenciled  6D6s  in  the 6E7  sockets. So there has been some substitution . That's ok  , I thought, I have a couple of  6D6s should  either  prove to be absent on leave. One  did so prove.A phone  conversation with a very learned person in the ChCh VRS ,though, suggested that there must have been a socket change ,  as the 6D6  fitted a 6 pin socket whereas a 6E7 was a 7 pin valve.On the walk back to the shed I  remembered  that the sockets are all original i.e riveted. Then followed some careful counting of pins on these sockets. Sure enough they are  7 pin sockets , original and riveted . So how come it's  6 pin 6D6s in  7 pin sockets? I began to think I knew.

Closer examination suggested that both 6D6s had been de- soldered from their original 6 pin bases and re- soldered onto  7 pin bases,as  you would if an original 6E7 was made from unobtainium.(Thank you David for your suggestion of  using a 6U7. I certainly have more of those than 6D6s and will probably have to follow that course). There were other clues. The soldering on the pins looked less than factory, shall we say ,  and masking tape 200 years old had  once  helped to hold the glass envelope to the bakelite base on one of the now suspiciously  faux 6D6s. 
Similarly there is presently a  " 75  " in the 7 pin socket which should be occupied by a 7 pin 6C7, also made from unobtainium. That 7 pin base  base  may also need to be "frankensteined" (thank you Steve!) onto a 6Q7 envelope as the 75 seems to be no go even with its cathode connected  to a cathode bias resistor (see note later) .
As suggested by Steve , when  made by Grigsby Grunow  the third pin on these valves was for their famous  sprayed on screen.The replacement valves in this set as found were protected from "stray oscillation" by Goat shields though not actually earthed through the usual Goat spring clipped bases.In fact not earthed at all.Perhaps that was the point of the spray paint. (See note later)
If you have ever seen or worked on a Majestic Model 118 set  you will realise why the previous repairer went to the trouble of frankensteining" 3 valves.It would take a complete rebuild of the set to change sockets. The set was made to fit into the glove box of a 1934 Ford V8,said box being roughly triangular in shape. Guess what shape the set is?

And on this set someone has helpfully sprayed the chassis with silver paint to provide multiple leakage paths.The original vibrator has long since passed on so a "normal" vibrator has been removed from its can and squeezed into the top of the  black box power pack after being tucked back  into its now gooey  sponge rubber hat.All wire joins are carefully insulated with that lovely tarry insulating tape we used to get before the war.The cathode wire for the 6C7/75/soon to be 6Q7 went nowhere. On the bright side the 42 does pass a signal and the old e.m. speaker has been replaced by a nice new p.m. substitute,which means less battery draw.So, watch this space! Thanks again for the replies and cheers.