Brief recap!: When Mr. Oswald (green arrow) leans to his right, we catch a glimpse of his reddish-sleeved right arm in the glass behind him (yellow arrow)
But there is more!: That lean also affords us a fleeting glimpse of
something else being reflected in the glass. Orange arrow:
What is it? It's too light-colored to be the back of Mr. Oswald's reddish shirt, or of his dark pants.
It's the flag. He's holding it behind him, in his left hand, and is a moment away from whipping it out and starting to wave it.
This also explains why we don't see the flesh of his left hand: it's covered by his body.