Spare prop question

[fatrap]

I recently came across a rebuild aluminum prop that I'd like to use as a spare on my 150 Mercury Opti. It's a 19 pitch but even though the owner said it came off his Mercury the spline off my Tempest 21 SS prop doesn't fit inside the aluminum prop. Sooooo...Either this doesn't fit or the aluminum has a different spline set up. Anyone know for sure?

fatrap