You know it's really maddening, I heard somebody on the forum saying that Nevis boot on the 606 last feels much wider than Dover shoe built on the same 606 last.
My understanding is that 64 last is little wider to accommodate thicket socks.
It's very difficult to determine if one should go up in width because if you do you might need to go down in size, for example if you are 10.5 E than you should take 10 F otherwise shoe might feel long.
Yeah wish I had a place to try 64 last in person. I just need a little bit of extra width on E normally and with thin socks it'd be ok. Want to wear medium socks with these so maybe E would work but probably going F to be safe. Little bit big better than too small.
Also iirc when the rosewood cc boot was posted a few months ago I believe a few said 64F was super big