Let me jump in with an opinion which is worth just what it costs you.

Be not concerned what load it was reproved for. It clearly is a black powder rifle, weighing less than 12 lbs. It wasn't made for the all-out .577NE stomping load. Evidently it demonstrated sufficient strength at the proof house to merit re-proof. That doesn't take anything away from the rifle, nor does it change the original essentials of it. The new proofs don't require the shooter to use the full-stomping load. Quite to the contrary.

If this were my rifle I would load and shoot it as a NFB configuration using black powder equivalent loads to duplicate original ballistics for which the rifle was made.

I can give an example from my own toybox. My James Woodward .450/.400 3.25" is a black powder rifle that was later prooved for the full nitro load. What do I shoot in this lovely rifle? Nitro-for-black loads duplicating original .450/.400 BPE ballistics. Sure it's wearing the nitro proof stamps. That doesn't change the rifle one whit, and it certainly doesn't take anything away!

http://www.rbsiii.com/collection/rifles/woodward_400NE/woodward_400NE_1.htm

So, have no regret of the nitro proof. The rifle is what it is. Use it as originally designed and intended.

Curl