If it's the same as a mark II, start pulling up at the right side of the grates. They will disengage from the shaker "rack".
The grates sit in cast channels - those channels are also removable. My grates/channels were packed in with ash from the sides, almost like cement. Once most of it was cleared out the grates could move side to side and was able to work them out. Spent the better part of an hour lightly prying up/down to loosen them, another hour chipping out packed in ash from the channels and the stove body where they sit.