Ed,
The 176 was made by the original H&R that went out of business in 1986, they didn't offer a different frame for their rifles and shotguns, they were all the same cast iron frame, H&R1871 started offering SB1 and SB2 frames in 1987, cast iron SB1 for shotguns/muzzleloaders, heat treated investment cast alloy SB2 for rifles, the SB2 10ga frame is basically an SB2 frame hogged out for the larger barrel with a larger firing pin.
Dunno about the M176 other than that.
Tim