Yeah, I have a '74 1-18, and I've never figured out what that "sanitize" thing meant, either! It certainly doesn't appear to do anything different or special during the rinse cycle...
Sanitize is to run a rinse with a cup of chlorine bleach (or some specific amount, it's stated in the instruction manual), for purpose of sanitizing the machine, I suppose after washing something particularly nasty.
"Sanitize", just like DdDoEs said, bleach or ammonia added during a rinse cycle after washing a ill family member's laundry. Cool Down was a series of one or two partial drain and fill (with cold water) and brief agitation before the complete spin between wash & rinse. For perm press & knitted fabrics. Had an automatic forced gentle spin speed on those cycles and user instructed not to use the fabric softner dispenser on these cycles.