That lip on the front looks suspiciously out of place and the way the "mortise" (if you can call it that!) for the lock looks like it was an after thought the box was built.
I'd start again with a new box myself if the owners want it locked. Or you could remove the lip, fill in where the and put one of those hasps on the front with a padlock.
Some of the larger pre WWII sets did come in wooden boxes similar to this and they also sold special boxes for enthusiasts to store their collections in and indeed some of them did come with a lock and key but I don't think this is an original box made by Meccano Ltd.
I think the locks used were the type one finds on old desks with drawers in them - i.e. two prongs on the lid with a socket on the box with the key locking the prongs in place.