You bring up some good points.
First, I do not think there even was such a thing as an 'outdoor enclosure' in 1959. I believe the NEMA designations came about in the early 70's ... and I do know we were well into the 80's before UL recognized them (UL had their own criteria until then).
Also note the mild corrosion. Panels of that time were often just painted; only recently have they been required to be galvanized as well as paint
I have seen a good number of rain tight Cutler-Hammer XO meter/dist panels(meter socket & 12 circuit main lug panel in one) of 1958 or earlier vintage as locally there were subdivisions full of them & a lot of them are still in service.
