Dave,...I'm not familiar with the slots on a Laser prop, but i suspect they are for ventilation. Here's how the Merc props work.....the "Vent Plugs" are removable plastic plugs that allow exhaust gas to exit quicker, thus allowing the prop to spin faster on your holeshot, which inturn gets your RPM's up faster and you get a much better holeshot. These are available in 3 varieties,....small hole,...larger holes,...and solid (no holes..just a solid plug)
These allow you to customize your prop to your boats load and your personal applications (skiing, light loads,..heavy loads, etc,..etc.) for best overall performance. Sometimes the difference can be amazing. My 901 runs great with 2 plugs removed out of the 4,.....i took a 3rd plug out once, trying to get an even better holeshot and it was too much......the prop would cavitate like crazy and my holeshot was horrible!! I put it back in and it made all the difference......so depending on your load and your requirements, you can tweek your boats performance with 1 prop....just by adding and removing plugs. Once your boat is up on plane the exhaust exits directly out of the hub as designed,....the venting features help ONLY with the holeshot. Hope this clarify's your questions,...holler back if you have any more,....best fishes!!