50 year old home probaly has plaster, even lath and plaster.
best bet is attempt to find studs cut gently test hole in wall check
for obstructions like water sewer telephone, firestops, etc etc.
dog doors are wonderful having installed a couple best bet is probably
under window, that elminates many utilities like water and sewer.
test hole is safest using hammer bash small hole in perspective area
look inside. chances are good you will be fine. best to start inside,
easier to repair damage if you foind unhally surprise.
the reason i recommend hammer it cant damage wiring.
we installed door then fenced in yard, no fdogt walking needed, no door
opening, etc etc.
dogs can go outside when they want, happer healthier furry friends.
self toileting too.
dog doors are wonderful, once you get one you will never go back.