A Brief Look at Paper Types
"Old" paper is anything that the first issue of documentary and proprietary stamps were printed on. There are varieties, and the best distinction between it and the later, watermarked paper is the absence of the watermark.
Some of the old paper has small, intermittent bits of silk in it. This "experimental silk" paper is also found on first issue revenue stamps. Any stamp on experimental silk paper will not have a watermark.
Silk paper was used for the second and third general issues of revenue stamps. This paper is characterized by the presence of numerous silk threads.
The reason for the use of pink paper is not known. The Boston Book says that it was only used by the National Bank Note Company in late 1877 - early 1878. Watermarked paper had been delivered by then, so this was not done to bridge the deliveries between the silk paper and that.
The white paper used in and after late 1877 contains a double-line USIR watermark.