Contrary to what Hallowell says, the demi-bloc (as originally defined by Pieper) is not the same thing as chopper lump. The demi-bloc is a special case of chopper lump - it is a chopper lump in which the two lumps are joined by use of a dovetail. NOTE: That is not the same thing as dovetailed lumps, in which the lumps are attached to the barrels by use of dovetails.
McIntosh discussed the difference between chopper lumps and demi-bloc in his discussion of the Winchester 21 in "Best Guns".
I used to own (wish I still did!) a pretty little Pieper demi-block 28 ga hammer gun. Upon close examination, it did in fact have chopper lumps, and the lumps were joined to each other by a vertical dovetail. The dovetail joint was almost invisible, but I was able to find it with some difficulty.
Jim Cassada, aka Seamus O'Caiside
