The blackmail letter thing explains a line in Gaudy Night, about Lord Peter being sent pills of which the first five "would neither make nor mar you" but the sixth was strychine, full lethal dose. And on coming out of jail the person "tries to blackmail him on the strength of the pill correspondence". Not, obviously, abortifacients, but the original pills were intended "to cure lassitude and debility" which presumably has a male code, just as "blockages and irregularities" has a female one.