on a 56, no turning - just a friction fit. Ive seen others make a yoke in a vise to hold the edges of the knobs, and pull down hard/swift on the lever. Ive seen people put a screwdriver or rod through the bolt hole on the bottom of the levers and whack it to get it loose.Please note that I have never done this (just painted the parts with the knobs still on - very carefully), but have seen it done this way and have had others say theyve done it that way.
always seemed risky to me, but...