Replacement 3/4-16 headstock. This is the type used on the Taig manual lathes and older mills. This is a popular DIY part as well. It does not come with a collet closing nut or pulley.
The newer mills, manual and CNC use an ER-16 headstock which is part #200-00ER. The CNC lathe uses a part# 300-00 ER-16 headstock which differs from the mill ER-16 headstock.