need prop help

[locknut550]

ive got a 1979 mercury 7.5 hp im wanting to change the prop seal. after having to cut the prop off im now faced with another problem. there is a retainer ring i guess thats what you call it. it looks like you would need a special 4 prong deep socket of some sort to get it off. anybody have any ideas. id hate to have to buy a special tool from the mercury place to use just once.