Tom,
Are you talking about the lock tab ring that goes on the prop shaft and is followed by the prop nut ? If so, there are two different kinds. The early style was thicker and had plain tabs that you'd knock into the grooves of the prop hub lock ring. You can use a pair of channel lock pliers opened wide to grip the prop shaft and edge of each of the 3 tabs to bend them outwards and then remove the prop nut. Mercury makes a pair of pliers for this type.
The newer style is the thinner one with the tabs bent up at a 90 degree angle and a small hole at the end. These you can easily bend outwards with a pair of needle nose pliers. Mercury doesn't make a special tool for these.
I think this is what you are refering to. If not please let me know and I'll get the answer for you.
Thanks,
Ron