The red sub 1680 with a MK II dial. They were notorious for tuning brown. There was a very specific characteristic dial that had this behavior.
If you see a feet first that is brown it is suspect. I may be wrong but I have never seen one in person.
The sweet spot was somewhere between 2.2 mill and 2.4 mil serials after that you move to the border of feet first +/- 2.5 mil case number.
Jump in Tyler the water is warm.
HAGWE
Bill