1149. This Portland has a few things in common with our Portland; there is a Portland Harbor on the north side; it's about the same latitude; It has an island that's not quite an island, and it is famous for being a certain color of gray, and at the end of it all, just like property taxes, you have a bill. And, speaking of ends, this Portland is Sine qua non.
Which Portland is this?
UPDATE: Stan gets the award: The correct answer is the Isle of Portland, in Dorset, in England.
The Sine qua non comes from the inference that most other Portlands were either named for this (Portland, Maine) or named for something that was named for it (Portland, Oregon). The famous Portland cement has nothing to do with our fair city (despite there once being a local firm called Oregon Portland Cement), but was in fact called such because the color was reminiscent of the color of the stone on the Isle. Quizzically, there is no actual city of Portland there. The nearest city of size is Weymouth on the Dorset shore, across the Portland Harbour from the Isle.
This island is physically an island but is also connected by a long sand spit called Chesil Beach, which, in geographic parlance is called a tombolo. Shaped like a great comma, pointing southwest, the cape at the end is called Portland Bill, which seems right when you look at the shape of the land, and inspired a popular BBC children's show whose star was called–of course–Portland Bill.