Just went and picked up a load of parts and this motor was fitted with a vacuum advance. I don't know anything about T distributors but I thought you still had to use the spark advance lever? Also what year oil cap is that? The block is a late 1921 but it's all a hodge-podge of parts.
It's replacement distributor, originally designed for a more modern car which used vacuum advance. Depending on the distributor and drive gear setup, the distributor may run in reverse from the design direction. If so the vacuum "advance" will function as vacuum retard if connected.
This is Frank Harris using my son's computer and not Bill. I Frank have removed the internal advance parts and cut the vacuum advance unit box off and epoxied the hole up and checked it up in a lathe and cleaned it up so you don't know it was ever there. Then wire down the advance plate or fly-ball governor so it will not move and then run it like you should with your spark lever. The modern distributors with vacuum advance are not calibrated to the rpm or loads of a Model T and never do their job correctly.
This one is pot metal and the bottom is all broken. I like coils anyhow. Just thought it was odd having an advance.