For this item, I recommend APEX, the expertizing service of the APS. I have only had good results with them for U.S. stationery.
http://stamps.org/Stamp-AuthenticationSince you are trying to find out if this is a cut square that is currently not included in the catalogs, you want to carefully construct your question to the "experts". Specifically, you don't want to put any thoughts into their heads that might send them off on a tangent unrelated to your question (I speak from painful experience).
Manila is not a color, but rather a quality of paper. The experts will be using a high powered microscope (to look at the fiber sizes in the paper) and other such tools that we don't have access to in our daily collecting lives.
You want to find out if the paper is manila. If you are lucky and they do find it is manila, then the lightbulb will go on when they write up the final opinion and they'll have to say something like "previously unlisted in the xyz catalog". Pick one of the two watermarks (19 or 22) and include it in your question for expertizing. That way they will pay attention to it and include some statement about it in their decision.
P.S. Don't feel bad when they tell you it's white. :-)