The Kalloy seatposts are reliable seatposts. This seatpost with ATB, or patent head, is 400mm long and has a diameter of 27.2mm. The model number of this seatpost is SP243.
27.2mm is the most common size for seatposts. This Kalloy ATB seatpost also fits other bicycles than ATB bicycles. The name ATB seatpost originated because the first patent head seatposts appeared on ATBs. The term ATB seatpost only refers to the way of mounting.