Edit : Okay, so I thought I was using the Butterick pattern that's above, but once I dug out the "real" stays, already cut out, I discovered that it was actually Simplicity 3635! While not as historically accurate as I would like, I think I can tweak the pattern enough to bring the final product up to standard. But, huzzah! It's not Butterick!
By the way, those panniers should be what mine end up looking like when they are finished, without the hassel of having to use a pattern. ^^