Possible laboratory dropper bottle?

[Patchwerker]

I am so curious to find out what this bottle may have been used for and what type of closure it's called. Never seen one like this before. Bottle is clear, round and small. Marked on the bottom to hold 1 oz., made in the U.S.A., has an A in a circle to indicate Armstrong Cork Co., probably made in 1941 if the 41 on the bottom is a date code. So, it would make sense it had a cork closure if Armstrong made it and there are no screw threads, but I can't understand the use of the ring that is lower on its neck. What was it's purpose? There's no cap to screw down to it, so it doesn't act as a stopper in that sense, so you wouldn't overtighten a screw cap.

Seams go all the way to the top. Lip finish is very even and flat all around, not flared or rounded. I can Imagine maybe a rubber dropper closure because it held such a small volume. I really hope you can figure out this mystery! See pix attached