All are short stroke 12 ft. lb. models. I do have 2 MarkIII's, but they also have MarkI pistons. I think 1 or 2 have the old FAC MarkII pistons (93 mm stroke) as well. These were stroked in between the old and the current stuff. You can use a softer spring and get easy cocking as well as 12 ft. lbs. I have 8 TX's and it's hard keeping up with the innards details. For sure, all have Maccari Apex seals.