I think your correct. I got this document in a lot with other autos many years ago.
During Andrew Jackson's second term Presidents stopped signing land grants, delagating the job to a secretary. If you notice on your land grant, underneath the Andrew Johnson is "By ------- Sec'y". In the earlier ones signed by secretaries the "By" is hand written. In both cases the secretary signs underneath the proxy signature of the President. You also have to be careful with land grants prior to Jackson's second term that were issued as copies for lost originals. These will have "signed" prior to the signatures written by hand. Hope this helps.