I forgot to add the 1/2" seam allowance all-round when I cut the back piece. No matter that I wrote in big, black letters on the pattern "add seam allowance!"
It's a rookie mistake. BUT! Fortunately I have a plan for fixing it. The doublet seems to rest all right at the shoulder seam without making the armscye too tight; the main problem is the one inch gap in front where the hems won't meet. I'm simply going to make wool strips to match the rest of the garment and insert them where the front and back meet on the sides. My first thought was to make them look like a bit of decorative trim, but I think that would draw too much attention to my error.
I'll cut them out when I cut out the skirting and epaulets, and it'll be just fine. I'll load a picture later on.