Great gun!! Congrats.!! I've got a 16-gauge NID No. 4E, but the only 16-gauge NID with a vent rib I recall off hand ever seeing was Chas. Askins' No. 5E that Elmer Keith had and wrote about for years.
A flyer introducing the NID showing a vent rib No. 4 --
You might want to post your pictures and info here on Shotgunworld --
http://www.shotgunworld.com/bbs/view...ccb7a8b2d494f5
and see if Walter Snyder chimes in with some info on your gun.