This is probably a 1940’s version of the even rarer original 1930’s strainer.
The original Sefco patent notes the benefits that include easy cleaning (no annoying wire mesh with bits of pulp, etc.) and a fit that works on almost any cocktail shaker, pitcher or glass.
Disc portion is 3″ in diameter.
We’ve only seen 2 others and one is owned by our Retail Manager! He uses nothing else.