Genuine Serotta CNC Alloy Seatpost Collar Clamp 
34.9mm / 1 3/8 Inches internal diameter for tubes that take a 30.9mm seatpost (they only made them in two sizes, I have the 31.8mm ones for 27.2mm seatposts for sale separately)

These are new old stock out of the old Serotta factory and some of the last on the market. They were stored loose in a bin and some were put on frames to test them so while they have not seen road use they may have some marks on them and are not necessarily perfect. I'm selling multiple of these so it may not appear exactly the same as the picture but very close. SEE PICS. 

I have loads more Serotta stuff - let me know if you're looking for anything specific.