BadlyWiredDog has a custom headset on his RC405, which i’m certain isn’t a 1.5 headtube. He’ll probably be along at some point today to point you in the right direction though.
It’s one of Saar’s, a -2? in a zero stack head tube, ID of 44mm. I have a Works one in a different frame too with a 1 1/8 standard head tube. Both spot on, slightly fiddly to instally, but Saar reckons there’s a 2mm alignment tolerance between top and bottom caps. It’s not that hard to line them up pretty closely.
Got the Saar set for the Pace because there wasn’t a Works Components option for 44mm when I ordered. Also Saar with do a zero stack lower cup, whereas works, I think, do it external, which raises the front slightly. I actually wanted to keep it low so I could run a fork with a slightly longer AC meassurement.
Brilliant things. Slackens the front, steepens the seat-tube angle correspondingly, so the bike still climbs properly.