Neville last looks in the wallet before its rediscovery
Date (happened)
c. 1939
estimated · possible · preferred
Reasoning
1969-11-21 (certain) minus a rounded thirty years gives circa 1939; the hedge 'I do not think' and the round number show this is his own imprecise decades-later recollection, not a measured interval, so no month or day is asserted and confidence is capped at possible rather than raised by the certain delivery date it is computed from.