Quite a puzzle.
The cover itself was three years after the stamp was issued. And on the opposite coast as the stamp was issued in Boston.
The ink of the signature looks like it's over the postmark, so it should have been in Chicago a couple days after the postmark. And the POD didn't generally like people signing envelopes while they were in transit.
But, since he lived in Burbank, I had a quick look for an event that might make the whole thing make sense.
A quick look showed that May 31 1950 was the 25th anniversary of Burbanks first park, Vickroy Park. Sooo …. maybe a celebration on that day?
https://www.burbankca.gov/department...-burbank-parks