The inscription combined with the symbol almost certainly is referring to an object which is associated with two patent dates May 2, 1871 and December 28, 1875.
In theory, this could refers to 1971 and 1975, though from the style and wear, this seems unlikely.
The U.S. Patent and Trademark Office maintains an online database which is publicly accessible for all patents issued. Unfortunately, for patents from this era, they are maintained only as pictures with limited information that is searchable digitally.
Using Google patent search, I have listed every patent that contains either date collated a list of possible candidates. Unfortunately, perhaps due to incomplete data OR a different concept of the "dates" on the part of the people who stamped this piece, I have not pinned down the patent.
Perhaps this stamp appears on a piece of equipment. (Please contact me if you run into this stamp!
