I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a decade of experience handling many different legal matters, including some similar to your issue. It is a pleasure to assist you today.
Due to the statute of limitations, it may be difficult to lay a claim to this instrument, if it was in his possession when he died. You cannot file a law suit for the rights to it, per se.
That said, you can assert a claim with his estate when it goes in for probate wherein you attempt to either locate or lay lawful claim to the instrument. You will have to be able to show some form of proof that the instrument was yours and was stolen or converted in illegal fashion. The estate will attempt to state that it has been too long and therefore even if your claims are true, you cannot claim it now (statute of limitations).
That said, since you were a minor, you can perhaps argue that when it was taken, you had no way to prevent it and have just now figured out that he died and want to assert a claim. It is a legal shot in the dark to make a claim, but it is perhaps the only way that you can at least get some sense of closure relative to this horrible thing that happened to you in your youth.
In short, you need to try to make an entry of appearance when his estate is probated and try to claim or find the information relative to this instrument.
Let me know if you have any other questions or comments.
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Best wishes going forward!