Pistons are rated for compression with a given CC combustion chamber. Just because a piston is rated at "10.5:1" means nothing, unless you have the combustion chamber size that the piston is rated at that compression for. Chambers could be anywhere from 65cc to 72cc. POssibly a little more. So unless this was blueprinted and MEASURED, it's unlikely you just "lucked up" and landed on 10.5.
Condolences on the loss of your grandfather.