Train Mail cover

It is amazing how this plain looking philatelic cover can cost me about 120 times the market price of the basic used stamp!! Similar covers like this are not uncommon but obviously sought after. The postmark is the more plentiful D9 with the month in 3 letters. There are no transit or arrival markings which would probably indicate this is a CTO cover.

 A couple of D9s off cover. They are not too difficult to find if one tries hard enough.

This is the interesting one. This clear partial postmark does not seem to fit D9 or D10. It is definitely not D10 as there is a faint 3 letter month of DEC. It is a bit bigger than D9. This is quite possibly a yet to be classified train mail cancellation.