I inherited some old pocket knives and most have considerable amount rust on the blades and back springs. What is the best way to clean off this rust. None are valuable other than sentimental value, Case, Frost, Buck, etc.

Soak in 50/50 acetone and transmission fluid. If you can get the steel parts off, put them in vinegar for a while after.

What are the slabs on the sides? A lot of 50's 60's knives had different plastic slabs, and the acetone will attack the hell out of a lot of them.
Vinegar works but slow, I would try Evaporust or equivalent first, as it should harm the slabs. Then hit it with fine steel wool, or really fine sandpaper.
As a Blacksmith, I have had a lot of time cleaning yp old tool steel.

I build only high carbon steel knives. Don’t leave them wet or put them in a dishwasher. I like Noxrust, light weight sandpaper and then re sharpen if needed.

WD-40 and steel wool

I use a very mild polishing compound and a cotton buffing wheel on my bench grinder.. Typically cleans things up very nicely (and quickly)..

don't store in leather sheaths, some are tanned with salt and will mess your knives up

I was told Flitz was a good option. Is that too abrasive?

Best rust remover/preventative.