This is a little off-topic from switzr1's original question but it's kind of relevant to the theme of the thread. I have been very confused about the SP, SSP, and DP designations on certain cards. I believe it's often mis-used, sometimes intentionally, by many in the hobby. I primarly collect vintage football so I don't have the same problems as modern-set builders but I do want to understand why a card recieves an SP, SSP or DP designation.
In researching why such designations exist I found this site by "nearmint" that uses actual printing sheets to analyze production stats of vintage football cards. After reviewing his site and the uncut sheets, the true meaning of SP, SSP, and DP became clear. Granted this was back before card companies intentionally short-printed cards in order to make you buy more packs.
Here's the link if your interested:
http://www.footballcardgallery.com/uncut_sheet/1969+Topps/
My two-cents is worth slightly more than a penny.
-- Chad --