They look identical from the top when installed. You will need to back the plug out to be 100% certain. If you have a pic of the rifle, or the two letter prefix for the serial number, I could tell you.
OEM H&R/NEF Breech Plugs. 5/8" on the left, 7/8" on the right.
Good luck with your new rifle, they are great shooters. .502" bores just like the Knight .502" Green Mountain Barrels. I've spent a lot of time behind the Huntsman and two Sidekick's, shooting Blackhorn 209 and other powders. This includes several OEM breech plugs, custom breech plugs, PR Breech Plugs, as well as the first to shoot Blackhorn 209 through the Metrics Unlimited prototype plugs. Both 5/8"and 7/8".
If you have any questions about the Huntsman, Breech Plugs, Blackhorn 209, just ask.
Congratulations on snagging one!