It’s an offer letter. Reply as follows:
Tell them you appreciate their hard work in locating you. That you knew your family owned minerals on both sides. Ask them which side you acquired the minerals from, and could they mail you a copy of the acquisition instrument for your records. Tell them you are interested in leasing, but that you would prefer to have the support documents in your possession first. Whether you ultimately choose to sign is up to you, but it sounds like found money.