The ones on Bill’s “Angelica” are “rain gutter” clamp on style if they are the same ones that I put on the car.
There are allen snub screws that hold each mirror in place. Due to the design of the CMC windshield frame, I drilled two access holes in the inner part of the channel to get the allen wrench at the snub screws.
That’s probably a better idea than drilling holes in the windshield frame channel.
My current “Pink MG” has an OEM MG-TD windshield, frame and mount. It can be folded flat (towards the radiator) as on the real TD’s.
It has a curved, slotted channel that the previous owner put a couple of Harley-Davidson mirrors on. They had threaded stems. The passenger side mirror sheared off. When I get the car done, I’ll replace the driver’s side one. It was a neat idea.