Brad, I have collected that there are 3 film cases on White Sands
Ground around the mentioned dates: one (by Charles Riggs) on April
1950, and two more (one by Floyd Fannon, another unknown) on May
I have the following references to back-up the two last cases:
"Saucers", Autumn 1958, pages 2 and 13.
Comprehensive Catalog of 1,500 Blue Book Unknowns: Work in
18, 2003, by Brad Sparks.
Projects Grudge and Bluebook Reports 1-12, NICAP, pages 81 and 84.
Loren E. Gross, UFOs: A History. 1950: April-July, Supplemental
2000, page 48.