2007 SP Authentic By The Letter

So currently, I am pretty obsessed at tracking these cards on eBay. But then I started getting confused. Now this may be a dumb question, but I didn’t find any answers to it online. I thought the idea of these cards was to collect them all to spell out the last name of the player. But there’s some letters that don’t work so what’s up with that? And also the number is either out of /50, /40, /25, etc. so does that mean there are only that many of THAT particular letter that is signed or just letters in general signed? What a confusing subset to collect. Maybe someone can enlighten me on these things. I also thought it would be pretty cool if these were all game used jersey letters, but I googled it and found on an old post on Wax Heaven that these letters are not game used and made specifically for these cards.

Where is there an “R” in Tim Lincecum?? At least “K” works because Strikeout is Timmy’s middle name.

EDIT: I think I’ve figured out the numbering after looking through several completed eBay auctions. I think the serial numbers mean that there is only so many of that letter signed only. That makes the most sense to me, unless someone else has proof of something else.