When Sydney wakes up in the Swamp he opens the door to walk out twice.
When Potter and Sidney are talking in Potter's tent, as Potter stands up and walks toward the door Sidney's legs are crossed and his hands are cupped on his knee. But in the cut to a close-up, Sidney's left hand is near his head.
In this episode, Col. Potter tells Sidney Freedman that he's 62 years old. The absolute latest that this episode could take place is during the first half of 1953, meaning that the lastest that Potter could have been born was early 1891. However, Potter stated in a previous episode that he lied about his age to fight in WWI. U.S. forces did not begin fighting in WWI until mid-1917, which would have made Potter at least 26 years old and therefore it would not have been necessary for him to lie about his age.
When Hawkeye and BJ are arguing with Charles in the Swamp, you can very clearly see their shadows cast by the stage lights on the walls and roof of the tent.
Captain Schnelker comes to teach the MASHers how to deal with phosphorous. S02E01 "Divided we stand", we see that they already have had experience with phosphorous, so they should already be familiar with it and having to treat it.
White phosphorus was used during World War ll by all sides. So dealing with the wounded from such weapons would not be new to the American medical corps. And Col. Potter who served in World War ll would not have been unfamiliar with these weapons.