There are two styles.
The pin style uses a small bolt (like 3/16) to secure the timing to the right spot on the gear, then six 3/8 bolts that pinch the gear to the hub.
The puck style uses a large offset puck that inserts into a hole machined into the gear, and bolts to the hub using a 3/8 bolt. There are four other 3/8 bolts that pinch the gear to the hub through the slotted sections.
The advantage to the pin is that you can rotate the pump 360* separate to the the engine, meaning if you wanted to, you can put your engine on TDC, put your timing gauge in it, and just turn the pump to the desired reading then find the pin hole that lines up, and bolt it down. The disadvantage is that the little pin likes to strip out, not line up perfect, or shear off if the other bolts aren't torqued enough. I personally have taken to getting the timing where I want with this style gear, then installing the small bolt loosely, then tightening the other bolts to torque, then torque the small bolt. It is a little annoying but its easier to adjust since you never run out of adjustability.
The advantage to the puck style is that the timing is absolute, there is no way the bolt for the puck is going to shear off and timing would slip. You get a solid hold. The disadvantage is that you only have about 9* of adjustability without removing the outside gear itself. This isn't a huge deal, since if you jump a tooth you gain 10*, but annoying if you have a front cover that the gear doesn't fit through, as you'd have to pull the whole cover if you wanted to adjust your timing more than 9*. It is also sometimes annoying trying to find the puck that fits correctly, and additionally you have a whole stringer of pucks that you need to keep track of.
All things considered I think I prefer the puck style, I just like how the 3/8 bolt seems more secure than the little pin. That being said I currently have a pin style, and it isn't an issue, it just happened to come with the pump I have. No matter which one you get make sure you use loc-tite on the bolts and torque them down. If they begin to loosen it will eat up the timing cover and fall out.
