My Parents also have a ‘56/’57 (can’t remember which yr) set which I used all thru Grade/Jr High/High Schools in the 50s & mid-60s.
I have two sets. One from 1952 and one from 1903. The older set is only a 3 volume set and there are no color pictures. There are only very small line drawings in black and white. they are leather bound and it says “cyclopaedia” on the spine instead of encyclopedia.