How do you value inherited shares?
I think you might be able to use the basis as of the date when the shares were physically transferred to you or the alternate date of valuation, 6 months after death of your grandfather, if the executor used this method.
What this effectively has done for you is to provide you with a stepped up basis for the shares that were passed to you. In effect, the basis would be equal to the prevailing share price as of the date of transfer to you!
This is great for you because now you have acquired the shares at almost FMV and if you were to sell these shares, you would presumably end up with very little capital gains and completely avoid capital gains tax altogether!