Mystery card.

This is a 1989 Topps Gary Sheffield rookie card and its a mystery to me.
As you can see from the picture, the card has been signed and hand numbered out of 1989 but the mystery is where did it come from? I don't mean where did I get it from because I remember when I got the card but at the time didn't notice that it was signed. I probably just saw it was a 1989 Topps and since I already had the full set didn't give it a second look. The card was pulled out of a pack I picked up at Walmart back in 98 or 99. It came in a pack of about 100 or so cards from mixed brands and years, I think it was about $2 and it the time I was going for quantity rather than quality. Back last year when I got back into collecting I came across this card sat in my common box and was about to throw it away because I thought someone had written over it. But then I thought why would it be hand numbered? and the auto looks pretty good to me too, a bit faded but still looks good.
The question I want answering is what product could this have come from? As I have said I picked this up in the late 90's around the time when autos were not commonly inserted into packs. I have done all the usual Internet searches for information but have come up with nothing. I did post the same question months back at SCF and got some good responses but nothing concrete. The general consensus was that it was either a fake ( probably, but until I know for sure I will consider it genuine), a buyback and inserted into a Topps product ( No evidence to support this as there would be information in Beckett) or possibly it came from a shopping channel ( This is probably the most realistic of the answers I was given, but still there would/should be information on this).
What do you guys think? Have any of you come across a similar card? If this is genuine then there are another 1988 of these cards out there. Any information on this card would be greatly appreciated and if anyone comes up with a definite answer they shall be rewarded.