So the trailer for that Jem Belcher film

The high guard is fine, based on the manuals of the time. Between Belcher's era and the late 19th century, the hands gradually dropped for some reason or other.

The hooks are questionable.

On the other hand, what's captured in the pictures and manuals is probably only a fraction of how people fought. Styles would vary by fighter. Prizefighting's ties to the Fancy helped preserve a lot of stuff, but it wasn't a subculture of rich, highly literate, artistically gifted people who were doing the fighting.

 

One of the things I'd pay attention to

Boxiana, volume 1, page 136, Jem Belcher vs Burke, round 6:
Sixth.--The best round that had been fought. As usual, Burke ran in full of spirit, and severe blows were exchanged. Belcher put in several severe hits on the head, neck and throat. They closed, and considerable skill was manifested on both sides in wrestling; but they both fell without any advantage.

I can't envision a hard punch (with bare knuckles) to the throat that doesn't inflict such damage that the victim can't fight no longer. And overall, hard punches that I've seen in the trailer are too risky, considering how fragile the fingers and the wrist bones are. It's one thing when you can use cloth wraps (bandages) that fix both the wrist and the fingers, decreasing the chance of them getting broken, like they do in modern ungloved fights, but back then, throwing a punch to the head with full weight of the body behind it sounds like the end of the fighter's career.

 

He was dominant in his prime, but his legacy is as much about his 2 title shots while blind in 1 eye.