We have 2 Metal rounds Antique items.  The one with the attached metal piece is older than the other.  This one measures about 3 1/8" x 2 1/8" across the main ring.  On either side of the 3 1/8" length the metal is thinner where the o ring of the attached piece has been in place over time - not sure if it was made this way or worn away.  The piece attached has a few threads at the very end - it measures about 3 3/8" in l.  It is completly rusted over.  The other O ring is round being about 3" across the center x about 3/8" in d.  It has a small amount of surface rust.