I don't collect Seated Liberty coins so I wasn't familiar with the known problems for any of those sets. My guess was "environmental damage" but "rusted die" makes more sense.
The obverse appears to have a little wear on the high points of the devices. The reverse less so but some nonetheless. I would grade it AU 55 and expect to straight grade by a TPG despite a couple small rim dings visible on the reverse. The Fortin Plate coin shown for the die marriage in the link is MS 63 by comparison. This is certainly a quality lightly circulated coin in my opinion and worthy of TPG submission. Congrats!
I'll make a wild stab at the grade. If there is an old cleaning that is not always discernable from a photo. Assuming there isn't, I like ms-63 for the 1849-O.
To clarify, I have noticed the color change at the knee, however, haven't seen any trace of metal disturbance or friction there. The color change could be caused by storage inside a paper flip envelope. Also in 2021 I bid on unsuccessfully an earlier Lib Seated with {what appeared to me} an actual touch of friction on the knee which was in a straight 62 holder. I was a bit conservative in pricing tho: hence the losing bid.