The 10 Apr 1884 date is on the death certificate, and was given by someone else, so may not be accurate.
20 Apr 1884 is given on Martin's WWII draft registration, and since he gave it, I'd go with that one.
The 1942 draft registration, aka "the old man's draft" was not for military duty, but was more for a man-power and skill database to help the war effort.
Dave