Your comment about the similarity to a shell jacket is astute. When I first got my Sheriffmuir doublet, a lady who makes reproduction Civil War clothing for local reenactment groups (and whose husband has recently gotten into kilt wearing), commented that it was simply "a shell jacket" with x, y and z modifications. Her assesment: "I can make one of those and I don't even need a new pattern!"

End result -- she has a very happy husband, who should soon have a very nice new doublet. :-)