All current production H&R/NEF stocks will interchange and fit SB1 or SB2 frames, most forends will fit, if not, a little opening up of the barrel channel may be needed for larger barrel contours, unless the 10ga is involved which is another matter from what I've learned here.
Can't tell ya if the internal parts are the same, other than pre-Marlin H&Rs didn't have a transfer bar. I have an early 1900's H&R 12gauge, it's at Dad's right now, I'll have to check it out to see if there are any similarities other than basic design, I know the forend snaps on, no screw.
The steel forend spacer fits all current wood stocks, the steel trigger guard needs to be fitted or the front pin hole in the guard won't align with the holes in the frame.
Tim