When I originally posted this little game, I said I would wait until February 16, 2023 before posting my list as a comparison point for entries offered by others. Is it February 16, 2023 yet?!

I apologize for the lengthy delay - this one slipped off my radar - but here's the list of differences I see (multiple have already been identified by others above):
1. On the Obverse: The inscription "UNITED STATES OF AMERICA" incorporates bullet separators between each word on the models - they were removed on the coin.
2. On the Obverse: A thinner font was used for "UNITED STATES OF AMERICA" and "HALF DOLLAR" on the models - it is noticeably thicker on the coin.
3. On the Obverse: Bullets are seen before and after the names "GEN GEORGE McCLELLAN" and "GEN ROBERT E LEE" on the coin but are absent on the models.
4. On the Obverse: On the model, the orientation of the stars differs from the coin. Two of the three stars at right are aligned straight up, as is the bottom star on the left. The other stars are angled inward. On the coin, all of the stars point inward.
5. On the Obverse: There are multiple differences on the coat of General McClellan between the model's design and coin's. On teh coin, the most obvious is the coat has two buttons, on the model it has three.
5. On the Reverse: The model shows more separation of the trees at left. On the coin, they overlap much more.
6. On the Reverse: On the coin, the inscription "THE BURNSIDE BRIDGE" is flanked by bullets. These bullets are not present on the model.
7. On the Reverse: On the coin, the inscriptions "THE BURNSIDE BRIDGE" and "SEPTEMBER 17, 1862" are smaller and better spaced vs. the model.
8. On the Reverse: The individual words of the inscription "SEVENTY-FIFTH ANNIVERSARY BATTLE OF ANTIETAM" are separated by bullets on the model - the separators are triangles on the coin.
9. On the Reverse: A thinner font was used for "SEVENTY-FIFTH ANNIVERSARY BATTLE OF ANTIETAM 1937" on the model - the letters are noticeably thicker on the coin.
10. On the Reverse: Bullets flank "E" and "UNUM" on the coin - they are absent on the model's design.
11. On the Reverse: Bullets flank "1937" on the model; on the coin, the separators are triangles. (@eaglebub: Interesting thought about the triangles representing gemstones for the anniversary. Isn't it the diamond shape that is linked to the 75th anniversary?)
I always find it interesting to see the interim steps that our coins take before becoming struck pieces!