I'm NOT a tax atty or CPA, so take the following at your peril. I think the basis would be the value of .866 shares as of the date of your father's death in 2006. Called a stepped up basis. For example if you inherit a home at your mother's death that she paid $10K for in 1955 and it is worth $2,000,000 when it is transferred to you at her death in 2008, then your "stepped up" basis is $2,000,000. If you sold it the next week for $2M, you would have no gain. However if she conveyed it to you the week before her death your basis would be $10K....its value as of the date she bought it. If you then sold it for $2M your gain would be $1,990,000.00
Fuch with the irs....if the value in 2006 is more than the value when you receive it, declare a capital loss.
In any event, even if your basis was zero, the maximum tax liability would be $260. Call the bank/holding company and ask what the approximate value of the stock was on the date of your father's death in 2006. Or make the assumption it was worth what you received and show no gain or loss.