The date 8th April 1940 corresponds with 29th Adar II 5600 unless the death
occurred after sunset in which case the date is 1st Nisan 5600.
5600 was a Jewish leap year which occurs 7 times in every 19 year cycle. In
a leap year an extra month is added. This is to ensure that Pesach
(Passover) always falls in the spring. As the Jewish calendar is lunar is
shorter than the Gregorian solar calendar an extra month needs to be added
to keep the two calendars synchronised. This is done by adding an extra
month Adar. In a leap year such as 5600 the calendar would include Adar I &
Adar II. In 5600 Adar I had 30 days and Adar II 29 days. It does look as
though a mistake has been made on the headstone.
A useful website is www.hebcal.com.
Bushey Heath, Hertfordshire