Welcome Will. The barrels are Parker D3 (3 Iron 'Oxford') which was the usual pattern for the GH.
Since the gun is > 120 years old you will of course want to have it checked by a qualified double gun specialist before use. Please see
https://www.shotgunworld.com/threads...-shoot.366087/
Let us know where you are and we might be able to recommend someone near you, with the interest and expertise to properly evaluate the gun.
If the barrels are free of pitting and the wall thickness is adequate, I would suggest avoiding today's pheasant boomer loads and stick to 1 1/8 oz at 1200 fps.
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