Fortunately for us, a screen-used Joker overcoat was auctioned through PropStore, and the listing featured some gorgeous HD photos of the overcoat.
As with his tailcoat and waistcoat, there were two buttons on each side, with the upper button spaced diagonally away from center.
However, unlike the buttons on his tailcoat and waistcoat, the overcoat buttons were large 4-hole buttons.
Curiously, although the buttons looked purple in the film, in the auction photos, they actually look a bit redder.
(It’s possible they’ve faded over time.)
Perhaps the most notable feature of the Joker’s overcoat was its contrasting collar; both in terms of fabric and color.
Here are some excellent photos of the contrasting collar from the aforementioned auction listing.
In the preceding two photos and the photo below, one can just barely see the collar’s fell-stitching along the gorge seam.
Although imperceptible in the film, there was a fitting dart toward the side of each front.
The buttons on the ends of the back strap appear to have been the same buttons as used on the front of the overcoat.
There also appears to have been a lower back vent a few inches beneath the back strap.
The Joker’s overcoat sleeves were, of course, two-piece set-in sleeves.
As with his tailcoat, the bottom of the Joker’s overcoat sleeves were vented.
Also as with the tailcoat, the sleeve vents were closed with four small buttons, though like the front overcoat buttons, they were 4-hole buttons.
These were 4-hole buttons, similar to those on the front of the overcoat but in a smaller size (more appropriate for sleeves).
Here is a lovely auction photo of the lower sleeve area, in which one can observe the sleeve vent, buttons, and their corresponding buttonholes.
And finally, I believe it worth noting that although the newly-reborn Jack Napier wore a dark purple overcoat during his confrontation with Carl Grissom, I believe it to have been a different overcoat than the one he wore during the film’s climax.
It may have been the same coat, but the sleeves and collar sure looked different to me.