I’ve a couple Vanson jackets in a 50 that are none too big. Got the J in a 52, probably could have gone with 48, but I’ll be using it like a parka, with layers, so it’s ok.
So yes, I’d size down in this if I were doing it over.
I’ve always had the J and F on my list, but after seeing your J, I knew I had to get the J first.
I recently got very lucky with a Vanson leather shirt on the used market. Incredible leather. I’m gonna let the crazy sale weekend to pass first and start chatting to them about a stock J in their motochap leather.