Delaney Strike Plate for 6" Flush Bolt, 2" x 1-3/8"

It's the strike, not the bolt. The Delaney strike plate is the brass plate that mortises into the head jamb or the threshold so the rod of a 6 inch Delaney flush bolt has something to drop into. It measures 2 x 1-3/8 inches. If you're replacing the flush bolt itself, this part won't do it, you need the bolt and this plate as the receiving half.

Usually yes. A standard inactive leaf carries two flush bolts, one at the head and one at the sill, and each one needs its own receiver. That's two of these 2 x 1-3/8 inch brass plates per inactive leaf. On a pair where only the top bolt is used, one is enough. Order per bolt, not per door.

Lay out to the plate itself: the Delaney strike is 2 x 1-3/8 inches, so the routed pocket in the jamb or threshold is that footprint, plus a drilled hole inside it deep enough to take the full rod projection of the flush bolt you're using. Cut the plate pocket first, drop the plate to scribe it, then bore for the rod.

Yes, it's a flat 2 x 1-3/8 inch brass strike and it serves either end of the flush bolt. Head jamb installs are a straight mortise into the frame; floor installs need the hole below it bored deep enough for the rod and, in a concrete slab, a dust proof arrangement, since a bare bored hole in a floor packs with grit and stops the bolt from seating.