No, it's definitely not an integrated 1" headset or semi-integrated one. I've dealt with those, and this head tube is nothing like that. The ends machined to a specific inside diameter at least 1" to 1 1/4" deep, and the "shelf" that is left is obviously not a bearing surface of any kind - in fact the machining reaches all the way to one of the holes made by either the top or down tube, so there's no way it could support an integrated headset of any type. It's just like a "normal" headset, just a weird size.
FWIW, the head tube outside diameter is about 45mm. The frame is a GT Legato, if that helps.
Not wearing a helmet makes me more aware of my surroundings. I find myself anticipating the hardness of concrete 50 or 100 feet in front of me, it's almost a zen-like connection between my face and the pavement.
I thought I might have some reducing spacers that would work but it turns out they would fit a tube ID tube of about 43mm to 44mm (there is a gap in the ring). They fit a generic 1 1/8 headset once installed but the OD doesn't seem to match up to any of the standards you listed. They came off a Giant XTC frame that has since been sold. I don't remember what headset I put in there but I think I got it from Cane Creek. Just another data point FWIW.
OK, a bit of luck, and I ran across a mention of this frame (GT Legato 3.0) having something called an "Intellaset" for a headset. And lo and behold, my frame matches the specs for one "Intellaset" headset produced by FSA.
The headset seems to be a one-of-a-kind that's no longer made.