"So, in a nutshell, DEDIC is LM+M-H as is common on Quebec boxes?"
Usually designated as L M- M+ H
It used to be more popular. As of late in Quebec one M row at "A" 440 then the M+ row.
If you see a description "equal tempered tuning" usually means no dedic.
I have a Quebec made box on its way.. when it gets here I'll see if it's dedic. About 10 days.
Consider this.. With dedic tuning if you want to play with only one M reed you are out of tune with every one else.
Often you will see Quebec boxes with non functional stops or only the L and H have working stops or no stops at all.. or often 2 reed boxes with fixed or no stops. These could be dedic as the idea is that MM are working all the time.
If you close off either of the M reeds you are out of tune with every one and yourself.