I've had several. Very nice bags but they don't last very long under heavy use(messenger work), the canvas tends to fall apart over time. It's not very waterproof either, especially once there are a few hole in the lining. One nice thing about canvas bags is that they don't move around as much on your back as nylon. They were also very simple and stylish, much more so than most other bags then and now, in my opinion.
Boots, your bag sounds very much like a breakaway bag to me, made by manhattan portage. Where did you get it? I've never seen a demartini bag with a plastic window.
They are the original messenger bag(nyc or otherwise), but the company has changed hands since their start. For a long time they were made in a basement shop on mott st, by Frank Demartini and his daughter. They also did canvas repair for sailboats. Mr DeMartini passed away sometime in the late 90s, and his daughter kept the shop open for a while but finally shut it down and sold the name to the guy who ran manhattan portage. He then made a series of "demartini" bags at their nyc factory, they used a thinner canvas and weren't quite as nice looking as the originals. He has since sold manhattan portage to some japanese people, but kept the rights to the demartini name. And now it seems he has this place in conn. making them for him. They are basically just the manufacturer, but they will sell to the public, but only if they have the bags around. I called them a while back, and they only had a couple of the small bags. They are happy to make more, but only in fairly large quantities.