Blaster, and Armorer77, I've seen the terms 'sled base', and 'sled bed' used many times before, some here and also in other places, (I'd imagine because this type of bed really does resemble a sled) but if you wanted to be historically realistic for the time period that we encompass in our discussions of artillery, according to the rules of the board, (in or before 1898) then the accurate term for a mortar's mount would be 'bed'.
DD's use of the word 'base', and RocklockI's dad's, and Corps instructor's use of the word 'baseplate' are absolutely correct for some of the the modern era's mortars, (as DD has already stated) their just wrong for the antique era that's the purview of our forum.