the medical attaché receives the cartridge — "The sole reason he does not…"
The sole reason he does not throw the unlabelled cartridge in the wastecan or put it aside for his wife to preview for relevance is because there are such woefully slim entertainment-pickings on his wife’s irritating Americanized tennis-league evening away from her place at home.
Context
This is "choosing your master" with the choice hollowed out to nothing. The attaché doesn't pursue the cartridge, evaluate it, want it — he plays it only because everything else on offer bores him and the evening is empty. The most dangerous object in the world enters his life through pure default, the shrug of a man who needs to be entertained and has run out of other options. That's the theme's dark joke about American freedom: the fatal master gets chosen not by desire or conviction but by the simple inability to sit an evening unentertained, the choice already made by the appetite long before any object appears to fill it.