NOS Shimano TRACK, Uniglide - Thread on 16T Cog 6 Speed - 1st position - DuraAce
This Cog is 5.5mm wide with a 3 mm shoulder spacer and designed for 6 speed but can be used on wider cassette bodies with spacers. ( The smallest sprocket on a Uniglide cassette was not splined, it was threaded. ).