Mystery solved!!

Thanks to a comment left on one of my posts I now know the origin of the Gary Sheffield mystery card!!
Fellow bloggers Swag of NYC Cards and Swag and The Chop Keeper of The Card Shop have said that it came from Score, Cherry Hill, NJ. I have done a few searches online and have found that this company was a leader of sports memorabilia and one of the players signed to them was indeed Gary Sheffield. I can not find the specific Sheffield card that I have but I'm 99.99% sure this is correct and I'm happy to close the books on this.
My apologies to the chop keeper, I obviously didn't read the full comment that he left and missed the part at the end where he said 'Cherry Hill'. Or maybe I didnt miss it and this was edited into the comment later. Anyway he was the first to name the company that it came from but Swag was the one that tied it all together. Swag may also have information on the Giants team ball which will tie up another mystery.
I mentioned that there was a prize for solving this and there is. I just need to find something for you both unless you want to share one prize and alternate it every six months!
I have two weeks off work now so this allows me the time to have a good look for something to send both these guys. So Swag and Chop Keeper expect an email from me at some point in the next week or two for your addresses and I can get something out to you. In the mean time if there any specific sets or collections that either of you are working on then please let me know.

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.