I would bore first so there is nothing to interupt the cutter ensuring a nice round hole and no risk of boring spreading it apart, then drill for the adjusting screw and eccentric rod and finally slit the single side.

If it did spring open a bit after slitting then the adjustment screw will close it up anyway

This is the opposite to how you would do a conventional strap that is split completely in half.

I just looked at the 10V that was my first engine, and I can see the slot in the extension strap is parallel and about 10 thou wide – I can't quite insert a 6" flexible rule, which is 12 thou thick. It's also not precisely radial, so I must've done it by hand. At the time, I only had a little Unimat 3 and couldn't swing the full casting, so I cut off the shank and made one separately. I'm pretty sure I bored the strap first and then slit it with a junior hacksaw. The fit of the bore on the eccentric cam is only to avoid excessive backlash – it's prevented from riding off by a little bolt running in a groove. The engine worked well from first assembly, and still does.