Bought a number of these off-clear glass "caps". They have 2 threads for screwing/locking onto the top of something... Above the threads you can see a black o-ring/rubber ring. No embossing. It's about 2 inches wide and 1 inch thick. Can anyone identify this and explain its use? Any info appreciated. Of the four units, two have very rounded domes, while the other two specimens are more ovular. These units were fished out of a bin of Lynchburg glass. Also note the palm tress in the background. Happy holidays.