The bolts with hollow knobs are referred to as "Type-III" bolts. They were introduced in 1952 on the Featherweight rifles, but almost immediately became standard on all rifles. The Type-III bolt is identical to the Type-II bolt, other than the hollow knob. Both Type-II and Type-III bolts have scope eyepiece relief on the handle lever. The Type-I bolt has a solid knob and a round handle lever with no relief for a scope eyepiece. There are a few other very minor variations within the types, but these are the three main bolt styles on pre-64 model 70s.
Type-I - 1936-1944
Type-II - 1947-1952
Type-III - 1952-1963
