See my edit above. Every steel frame my shop has in stock right now that takes a 27.2mm seatpost has a seat tube with a 30mm OD at the top. Not "most". Every. Single. One.

If you knew the number of 28.6mm seatpost collars customers buy from Retrogression, then immediately send back in exchange for 30.0 because it didn't fit whatever steel frame they have, your head would probably fall off your shoulders. We've actually considered not selling that size at all.

Fun fact: Even the beloved Kilo TT - which takes a 26.8mm seatpost - needs a 30.0 collar.