It's from the afternoon workshop at Newport 24/7/1965.
My research suggests it was performed immediately BEFORE 'If You Gotta Go, Go Now'. So we had three songs performed in this order:
1. Tombstone Blues
2. If You Gotta Go, Go Now
3. Love Minus Zero / No Limit
Murray Lerner footage in 'The Other Side Of The Mirror' starts with 'If You Gotta Go, Go Now'
How do I know this was performed first and not later or much earlier? Simple bit of detective work.
Look at these two shots - the first is from 'The Other Side Of The Mirror', right as Dylan is starting 'If You Gotta Go, Go Now'.
Notice the man with the destinctive white beard and hat standing up to the left of Bob Dylan. He has his jacket in his hand. You can see in later frames that he appears to place the jacket over the chair behind him and sits back down.
Now look at this frame, from the 'Tombstone Blues' WDR footage -
The same individual can be seem to Bob's left, but he appears to have his jacket on.
Also, if this isn't Murray Lerner's footage, then both cameras/cameramen were at least in very close proximity and recording from the same angle.