Jack, make sure you explore extensively, that is why we are here!
The sideplate, also called trigger plate and a few other names is a fitted item, but you can find sideplates on e-bay.com very often, plus several members sell parts on the forum.
Having a mismatched sideplate is a bad thing as a collector item, it brings the value down a lot.
Many WW1 Lugers were used in WW2, as they kept using them through WW1, then the Weimar period and then WW2. So, not a big surprise at all.
I collect stories, of guns brought back, and also pictures of the guns.
Ed