One of these helmets is signed by Chuck Noll who coached the Steelers from 1969 to 1991. Although his playing career was done with the Cleveland Browns, he became one of the longest and "WINNING"est coaches for Pittsburgh. His expert recruiting and skillful coaching created the formidable "Steel Curtain" which led to four Super Bowl wins!
The other signatures are all part of that Steel Curtain.
Full list of signatures
Mel Blount, Joe Greene, Jack Ham, Franco Harris, Jack Lambert, Chuck Noll