What government issued the grant?

I work with legal documents for a living so I can chime in here to some extent. If a document states that something is to pass on the heirs and assigns (or one of a dozen different legal phrases) then yes, that carries weight in the eyes of the law. It would be important to adverse the person who reserved it in order to make sure that they don't convey it out and on down the line (i.e. Grandpa reserves it, so you look to make sure he didn't sell it. Look to make sure dad didn't sell it etc) I don't know how the whole republic of Texas thing would work, but without reading the documents, this is, generally speaking, how it would work

I do hope a historian will chime in with information on how real property was dealt with and contracted, but if you want to know about something specific for your claim you really should talk to a lawyer.

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