Thanks for the input!
The fittings certainly look similar to the pictures I found for Parker O-ring/grab ring tube fittings. However, the spacer-ring/o-ring used in the Parker fittings looks to have a different shape from the similar hardware
in these existing fittings. However, I guess this could just be age.
I will buy a few to see if they match.
In any case, all of these fitting probably need to be replaced with something more up-to-date - but for now I just want to fix a couple leaks