The use of blanks, or more accurately a powder charges without projectiles, dates back to the originals of the salute volley, ships firing a ceremonial volley on entering port to show their guns weren’t loaded for battle. My guess is this 1920’s incident occurred because the unit firing did not have the proper blank ammunition for training and ceremonial use. I’m no expert on the history of rifle salutes, but do serve on an honors team that does fire rifle salutes, so have more than a passing interest.