Well I guess it could be. Sometimes glass things show up with out identity. Since it is milk glass there were a lot of cosmetic containers for women, that would require caps like that. RED M.
DISC IMMERSER TUDOR ROSE SNOWFLAKE MASON FRUIT JAR LID INSERT MILK GLASS ...
Just found one for sale on this site: http://www.antiquesnavigator.com/d-823902/disc-immerser-tudor-rose-snowflake-mason-fruit-jar-lid-insert-milk-glass-1880s.html
They were used and swapped around to more that a few jars. They also are sought after to complete a some jars so there is there is a demand even though they aren't that rare.
So many people don't ask the question and chuck it at a "what's it" so they can get rarer with time.