I've been in printing for 20 years. The material that I know that is used in the window of envelopes is either Poly (EZC), Glassine, or Crystal Clear (Cello). This is according to one of the larger envelope makers in the country. I've never heard of Tamarack. Below is what I found about Glassine which is a paper and is what I believe you are talking about.
Glassine is generally translucent unless dyes are added to color the paper or make it opaque. It is manufactured by a process called calendering; after pulping and drying, the sheets are pressed through rollers such that the paper fibers flatten out facing in the same direction. In order to maximize the smoothness and thinness of the paper, glassine must go through this process several times, and so is referred to as supercalendered.