A few years ago, I was given a Swiss feeler gauge as a gift. The data output of the mechanical clock was cut off. At that time I had attached a holder with a Ø 4 mm pin, which only allowed the gauge to be clamped.
The dovetail had an unusual dimension.
I ground a dovetail cutter from a Ø 4 mm graver blank. The narrowest point of the cutter is just 2 mm in diameter.
I milled a 3 mm groove 2.5 mm deep into a piece of Ø 8 mm round bar and then milled the dovetail in tenths of a millimetre with the graver I had made. In between, I checked from time to time how the dial gauge fitted.
The shaft was drilled through with 4.1 mm and an M 6 thread was cut approx. 15 deep at the end. A piece of the graver material (4 mm HSS turning steel) was cut off and ground flat on the graver grinding machine. This serves as a pressure piece for fastening.