From the 70's?
Looks to be 1/2" nominal. Normally we replace with 3/8 compression at the dishwasher and replace the 1/2" compression stop with 3/8"
Or you can see if you can still find a 1/2" compression fitting somewhere.
I pulled an old dishwasher the other day and installed a new 3/8" stop for the dishwasher installers. Often people will pay for an install at the store where they buy the dishwasher, but when they come out, if it's not a currently standard 3/8" compression, they unload the new dishwasher and leave.