Have you heard of the 1997 SP Authentic Sign of the Times Silver version? It's okay if you haven't, I don't know much about them myself and I've only ever seen a few for sale. According to Beckett, "Foiled and non-foiled versions of some cards were mailed as redemptions. While some player's cards have been found in both versions, others have only been reported as non-foiled." The problem is, every version I've seen I believe to be a fake. Here are the images that I've collected over the years.
Okay, so I haven't been able to find regular examples of Curtis Martin or Jerry Rice... but it doesn't take a certified forensic document examiner to compare the differences between the base version and the silver version. The only one I believe to be real is the Jerry Rice. I can appreciate athletics becoming fatigued after signing many autographs in a sitting and their signature having slight variations, but not wholesale changes that are shown above.
My guess is there was some production error, delay, or change in heart to get the foil versions out to the athletes or several real versions could exist in such limited quantities that they are rarely seen. But in any event, additional copies made their way out of the factory and into scammers hands. I wouldn't purchase any silver version unless the signature looks exactly like the regular version.
As a side note, Marino also has a fake regular version (middle). Surprise!