I'm a little confused.
Is it a through hub or non through hub?
If its a Through hub I think it should be a Merc prop which is an 8 spline.
It might have a Merc lower on a Force power head.
9" diameter. If its a Force I believe it would be a non through hub pin drive.
You might go on the Chrysler/Force forum see if they can help.My book only goes to
89 and the Force is a pin drive.Force started to use some Merc parts circa 1990.
But As near as I can tell the Mercs are 8 spline.
Try Marine Express@ 1-877-600-2966 I found this through a google search
"1990 Force 15hp prop". Actually came back to iboats someone looking for 8 X 6
15 hp Force prop didn't check the year.