I haven’t found an answer, yet. I looked at a special publication by the old Texas Cartridge Collectors Org. entitled 20 x 110mm Oerlikon AA (no date), information compiled from Bill Woodin, Robert Leiendecker and Robert Eddy with drawings by Robert Mellichamp. In the section on US 20mm Oelikon ammunition they show over 25 different headstamps of which 10 show the US and anchor, with dates of 1942, 1943 and 1944 and various makers. Lots of other info but no explanation of why some cases have the mark and some cases do not have the mark. Something is bound to turn up eventually.