I'd bet a bushel of enchiladas it's a Dura-Ace UA-100 headset from the 70's. I looked at your photo and noted the proportion that the races themselves occupied to the grooved outer portion. Those grooves were a clue. Those frames appeared to only come with Shimano so it was either a 105 or DA. It only took a few minutes to find this :
https://www.velosaloon.com/products/...read-from-1978
You'll see this photo that shows the bottom side of the crown race, marked with 26.4 and two letters which are possibly date/batch or some other code :



and from here you'll see the next photo that shows the top side the crown race with the same outer grooves around the bearing races themselves, the same as your posted photo.
https://www.velosaloon.com/products/...read-from-1978



Further confirmation here :
https://www.ebay.com/itm/304167775054


The only thing that doesn't quite match up is the bottom of your CR pic appears smooth while the one in the pic appears grooved, it could be the lighting of your pic however. The ebay listed on though shows a smooth underside, so different years/batches of manufacture.