How do I measure flywheel to see which one I have??????

How do I measure my flywheel to make sure exactly what I have. It does have a 10.5" clutch that was run on it. The measurement between the holes drilled around the edges is 11.25". Does this sound right for a 10.5" clutch?
I do have an extra 10.5" flywheel for a SB 273-318-340 motor in garage if you want I could measure it if you would like....
Let me know,
It's for sale also.
It's for Internal ballanced motor, it will not fit a 360 though.