Ouch! The space that had been carved out of the door frame was also a mess, so we took the brass strike plate from the linen closet door (which strangely has no latch anyway), checked its location with the latch (using a smear of mustard!), measured what it needed and filled the rest of the carved out space with wood filler. Sanded smooth, all it needs now is a coat of paint or two and then we can screw the plate on securely.
So many new things to learn!
Below, the door terminology image is from http://www.hardwaredetailed.com/.