That doesn't seem like the usual Winchester date stamp. It usually is all digits:
The first one or two digits are the month, and the next digit is the last digit of the year then a space followed by the day of the month.
Examples:
126 30 = December 30, 1936 or 1946 or 1956
80 23 = August 23, 1930 or 1940
Your box is clearly pre-WWII, so it was probably loaded May 10, 1939. "A" may have been a loading line designation.
Western always used a different alphanumeric dating system, and Winchester changed to the Western system sometime in the late 1950s.