Records of transfers of products/businesses

Question

I just found among some old family artifacts a "trade card" for "William Johnson's borax soaps", a product that seems to be the precursor to "Johnson's foot soap" still sold today. My g-g-grandmother was raised by her great-uncle William Johnson.  I see that the patent on this process for manufacturing soap was granted in 1874. Between the censuses of 1860 and 1870, William Johnson had moved to a different state and changed his occupation to "manufacturing", so the timing is good to be his patent. The trade card indicates that by maybe the late 1870s, the soap was being manufactured by Thomas Gill.  In 1880, William is back in his hometown listed again as "farmer", so I presume he had sold his product to Gill.  It seems that William created a new soap and started selling it, then teamed up with Gill for aid with running the business and eventually sold it to Gill.

My question: how do I locate a record of the transfer of the patent/business from William Johnson to Thomas Gill?

Answer 1

Answering my own question:

My wife is a paralegal and used her tools to access the Patent Trademark Office (PTO) database.  She says that a transfer of a patent is called an "assignment" and that the only way to see records of assignments this old (1874, in my example) is to go to the National Archives.  The webpage for the PTO collection at the Archives is here:

https://www.archives.gov/research/guide-fed-records/groups/241.html