The yarn description and needle size suggests something like a light DK, so that's attempt 1. It's not quite there yet, even with a needle size up it's still around one stitch off, and slightly too soft for this purpose.
I may need to add a few stitches at initial cast on to compensate. But it will do for bringing this pattern alive and figuring out how it works, and then for the next version I can focus on finding a slightly coarser, more wooly yarn.

Six months later, still doing endless moss stitch. As all the other patterns I tried from that time, the knitting isn't tricky and is relatively fast, designed I think to be done while doing something else - walking, talking... Also typical is the amount of yarn needed for the sleeves, and knitting fabric rather than whole knitted items. It's taken longer because I got bored and knitted a few other things alongside.
Edit: typo

It's sewn up, almost done, just missing the final three pieces. And in the end, it wasn't the sleeves, but what feels like miles of belt knitting that led me to putting this down in spring.
Maybe #FinishOrFrogAlong will be the motivation I need?
#knitting

@marthasterias Beautiful color and work. What are those triangles for?

@KarenStrickholm thank you :) the triangles are the lapel part of a notched collar+lapel. They are attached to the bottom edges of the u- shape above them and then along the front edge of the sides.
Having said all that, I'm not convinced about the collar yet. I've knitted it twice thinking surely I made a mistake, but this is what the instructions say. Part of the joy for me of bringing these early knitting patterns to life is creating something real from an idea!

@marthasterias That's crazy. I can see the collar buy the triangles, which way do they point? Are they in the front, lying under the C shaped collar? Hard for me to visualize. Curious!