WWI Officers' Mail Stampless Cover With Censor Stamp

I only have a few of these stampless covers dating back to WWI, so I'm not really sure what to look for in terms of special markings.

I assume this was a common type of cover in its day (11/12/1918) and would assume the American YMCA offered the stationery gratis, but I haven't seen too many of them designated "Officers Mail" and with a clear censored stamp, so I thought I'd share it:

I've had several like this before. Usually they are marked soldier's or officer's mail. I've seen one marked nurse's mail. It's a nice cover with a clear APO number - 755 and censor number. There is literature about the APO numbers out there and with some research one can usually peg the military outfit and where they were located given the date. Chatham, NY is just about 30 minutes or so south of me.

I believe APO 755 is from France. I haven't looked into it in great detail, though. Nevertheless, it's interesting especially now that it's nearly 92 years old.