I used to have a Cherokee friend in Oklahoma who had an original land patent on 160 acres. Sometime during the 30's, (I don't remember the exact date) one of his ancestors mortgaged 80 acres, and subsequently paid it off over the course of years. He still received a tax notice on the mortgaged 80 acres every year, but the first un-mortgaged 80 acre parcel was not taxable. I saw all his records about 15 years ago, and the original patent was still intact at that time.

Just an interesting sidenote to the thread.