Don't know if these will be of any help in determining authenticity, but these are Steinbrenner-signed baseballs offered for sale in the Yankees' team store. Presumably, authenticity is not an issue with these.
(Photo taken August 30, 2006. © Gary Dunaier. Link to upload on Flickr.com: here.)
(Photo taken June 21, 2008. © Gary Dunaier. Link to upload on Flickr.com: here.)
This last one is on display as part of the "Ball Wall" in the Yankees Museum in the team's present home ballpark.
(Photo taken August 11, 2009. © Gary Dunaier. Link to upload on Flickr.com: here.)