It is most likely an AGP pro slot, there are two versions of agp-pro, the differences are related to the total power available to the the agp card, and different voltage "keys". The voltage keys are little cuts in the video card that line up with posts in the agp slot. Check out the wiki regarding the different AGP slots; scroll down to the "compatibility" section and have a look at the diagram, it shows the different variations of AGP slots, with discriptions of what they are.