Licensed PE, been looking at cracks in structures since the 1980’s.
The hard thing about bowed wall surfaces is that I find that workers don’t build walls plumb in the first place… so a bowed wall condition could be the original as-built condition, without any forces causing it to bow later. It is a piece of the puzzle though. To be included in all the clues available to you when inspecting and finding cracks.
Thin hairline cracks and narrow cracks in cementitiuous materials (brick, block, concrete, mortar, grout, etc) less than 1/8" wide in buildings that are over 20 years old are almost never structurally significant, UNLESS there is evidence that the cracks formed recently, like in the last few years.
In my humble opinion, Your first instinct was right on. The cracks likely formed in the first few years (in the 1950s), and they have not changed in size since then. That is the definition of stable.
In addition, if there is no evidence of structural distress anywhere else in the home, in the basement or at the floors above, as you said, then it is VERY unlikely that there is any active or even past settlement going on. If the crack is only 1/8" wide in 50 years… Whatever forces or conditions that caused it to crack have stopped, so … Patch it, and call it a day.
Inspector TIP:… remember there are numerous causes for cracks forming BESIDES “settlement”. Try to avoid using that word in association with cracks…unless you can prove settlement is or has happened. Using that word implies that you KNOW settlement is or was going on, now or in the past, and that the crack formed from settlement. If it formed from shrinkage, or thermal expansion and contraction, or frost heave, or corrosion of embedded items, or whatever, then “settlement” had nothing to do with it. Just call it a “Crack”. Let someone else figure out whether or not anything is or was settling.