I need a couple of mortise locks to replace some existing ones (I am a new
owner of a house). Looking in the Screwfix catalogue (page 126) the two
options are 2 1/2" and 3", what does this measurement refer to I presume the
height of the 'innards, but how do I know how big the visible plate is. I
do not mind chiselling out for a bigger one but would like one to at least
fill the recess left by the other ones. If it helps the original ones are
Chubb 5 Lever.

TIA

John