As Jeff pointed out it's a Hyperglide Freewheel (not cassette). These existed for a very short while and ushered in the move to freehubs and cassettes. When Shimano introduced Hyperglide back in the freewheel era, they had to do away with threaded on sprockets, since there was no way to ensure their orientation. So all sprockets including the first slid onto a splined body and were retained with a lockring.

The shift gates and lock ring are the tip off.