Change June 1951 session to 4 May 1951

From EC:

Session DE5121, dated June 1951 in ellingtonia.com. According to TDWAW.CA, the session date should be May 4, 1951, based on DEMS Bulletin 2000.2 p. 19, 29-30. Such date is also accepted, with "?" in filmsgraded.com.
DEMS bulletins can be found in internet, through TDWAW.CA or other paths.