The Heritage Model shows up in "BLACK POWDER GUIDE", Major George Nonte, copyright 1969.
"HERITAGE MODEL: Top of the (Numrich) H & A underhammer gun line, fitted with walnut fore-end and brass butplate and patch box. Barrel is 15/16" across the flats. 32" long, available in .36 or .45 caliber, uniform or gain twist rifling. Hooded aperture front sight, elevator leaf rear, and long-range aperture tang sight. Weight 8 1/4 lbs. in .36 caliber, 8 lbs. in .45."
Most (if not all) Numrich "H&A" underhammer rifles use a .245-32 threaded nipple although if the nipple hole in the barrel has a bit of wear on it a 1/4-32 threaded nipple can be used.
A M6 x .75mm nipple will often screw into the nipple hole because although the thread is actually a 34 thread/inch the undersized 6mm (.236) diameter body will allow for the thread pitch error. DO NOT USE THESE M6 x .75mm NIPPLES IN YOUR GUN. THEY MAY BLOW OUT WHEN THE GUN IF FIRED.
On another issue, the Firearms Research Forum area is for original guns made prior to 1865 so I am moving it to the Percussion Forum.