[frankbmd]

For what it's worth, I worked at Wrigley Field in 1977. I have a similar ball signed by most, if not all of the players. Mine is also a Spaulding Ball (Sewn in Haiti). The Chicago stamp is the same font but mine also says Cubs below (but far enough below that it could be the same stamp with poor centering on the example shown.

There were always boxes of these balls in the clubhouse for players to sign before and after games and during rain delays. Once the balls were filled with sigs they were sold at the stadium (Mine was a freebie). Unfortunately my ball has very few autographs of consequence featuring immortals such as Bill Madlock, Rick Monday and a rookie autograph of Bruce Sutter.

I would infer from this that the OP's ball contains legit authentic autographs. Mine is less faded though and easier to read. Looking at it for this post it is remarkable that virtually all of the signatures are quite legible (pre chicken scratch era).