1851 Imperforate Paper Types?

I have a Sc. 12 on thick paper that has long fibers added in. The fibers are strewn into the paper, so it's not something from the envelope it was adhered to:





It kind of resembles a mix between silk paper banknotes and silk found in 1st issue revenues in terms of size and color.

Silk banknotes are the same blackish color, but are very finely chopped up:





Silk 1st revenues are of similar size, but are different colors:




According to Hahn and Neinkin, there have been imperforates with unique paper types that are probably the result of Toppan, Carpenter buying paper from the open market.
Does anybody have any experience collecting these?
Seems like something fun to hunt down.

There's also this little guy that looks like a small seed, or bug of some type:

My apologies.

imo the fibers are most often from the jute or sisal bags that were added along with rags into the mix. they appear as dark brown or black rather than blue or red of the silk type experimental or regularly issued papers. they could also be shreds of tree bark.

Greetings Stephen
May 2023 APS magazine had an interesting article on papermaking, by A J Valente. He states "the goal of my collection is to obtain some postal history from every papermill in the United States". In the article he goes on to describe paper manufacturing, as pertains to stamp production. he also dispels some widespread "truths" that have heretofor been accepted. His book "Rag Paper Manufacturing in the US. , 1801-1900 is available as a ebook. All of this from the article, I have no expertise in paper types. Stamps in the early days were printed on "security papers" and these are numerous types, made by different manufacturers.

regards mark
edit to add manufacturers

Paper made before the 1920s-30s was often changing and inconsistencies were the norm. Because of the ephemeral nature of the raw components used in the production of both ink and paper, making absolute or definitive statements is pretty dicey.

I my opinion, the 'truths' will continue to change on these topics far beyond our own lives and that is something people like A J Valente did not seem to want to accept (which is why he was removed from this site). I respect his research efforts but think his effort to "obtain some postal history from every papermill in the United States" will only provide a simple snapshot into production which was changing day-to-day, week-to-week, and month-to-month. (For example, see excellent remark that m and m made in his post above). All kinds of things went into 'rag' and other raw components that made up the paper slurry of those times.

In my opinion, anytime I read something on paper for this era in US stamp production that is presented in absolute terms, I get very leery. Today's 'truths' can often become tomorrow's 'myths'.
Don