Cel with DP in corner instead of a Sequence number

Usually I see a letter-number combination in the upper right corner of my cels. I have one that has the letters "DP" marked instead. This is not a Digital Paint image, which is what all of my searching has turned up. Anyone know what the DP stands for. This is an AIC cel for the show Magical Project S. This is a link to the gallery item: http://animeflash.rubberslug.com/gallery/inv_info.asp?ItemID=379969

I think it is a backwards '9'

I have a few cels like this, too.

Yes, for some reason many Japanese write the "9" in mirror image. "19" often looks like "IP."

I have a few like this as well, it is most definitely a 9.