I know "Mudsill" as a regional term here in the Nothern US. It's completely the same as sill, just refers to horizontal timbers that rest in the mud so to speak (ground contact). I find it often used by the elder generations. No offense NH!!
I guess it's one of those varriations, like "crow's foot" or "birds mouth"... same reference...
NH:
That tenon gauge reminds me of something I use to make canoe paddles with: a notched board. Mine is made of plywood w/ an 1 1/8" throat to measure shaft diameter on the paddles as I use a spoke shave.
