Thank you all for the advice, especially the correction that it would be C#, not D, if tuned up rather than down: that certainly says something about my thought processes at the time.
I have appended the tuning chart for reference, should anyone else have one of these to figure out (maybe someone knows how to place it with the other tuning charts on Concertina.net. Now, I am not certain, but I think this layout suggests a 'G' instrument. (When playing it, it falls beautifully, almost intuitively into the minor.
It certainly has been modified, very professionally, but nevertheless modified from a 37 button instrument, all the original holes, including the breather holes, have been filled, ie air release vent, reed shoes modified, face plate guides added , right down to the air release button, which has been swapped to the left, having the air breather pallet kept so as to cover the two reed shoe holes. These shoes had to be displaced to elsewhere in order to hold the low C#. I am left wondering if it is not a early, Jeffries Bros experimental duet I have, prior to their going into production.
I may play it for a while to see if I can get the hang of the thing before making up my mind. Possibly the quickest way to change it would be to make two new reed plates, be a dashed sight easier than digging out the filling, and shimming down shoe slots. (Anyone out there got a left and right Jeffries reed plate -- without the reeds -- they would like to sell me?)
Thanks for all the help.