Industrial grade quality ratchet handle with 10mm (M10) steel female thread 17mm deep.
Of course it is not a ratchet but a type of spring loaded clutch. You pull back the lever to release the clutch and position the handle to get it out of the way when tight. They are sometimes called Bristol or Kipp levers
This is well made and very strong. Much better than the typical handle you find on Far-Eastern made lathes. The all metal ratchet and 78mm handle will give years of good service.