help on old lid

[kingsporttn]

ok first off im new to the site and just learning about it. but i found a old cap of some sort about the size of a quarter and on the top it says .always place SMA tightly in cup. about 4 inches underground on a home site built in 1820. the metal was was silver colored and no rust on it or anything. cant find anything on the computer about. picture later on maybe. thanksss
also anyone around kingsport tn that has a club or anything let me know love this hobbyy