Issue #31 was the final edition of America's Best Comics published in 1948. Miss Masque appeared from #23 to the final issue. In this story a Kentucky Derby jockey and a horse trainer become greedy and want to steal the derby winning horse from its owner. They murder the owner, making it look like an accident, but Miss Masque is onto their scheme. She follows them to the stables, is spotted, KO'd, tied up and put on the horse which is then whipped to make it run wildly -- thus throwing her off and killing her in the process making it look like a riding accident. She manages to get free before the horse throws her, and jumps to safety, returning to defeat and arrest the bad guys. Pics below show her KO'd, tied up, and put on the horse. Quality is not great because these are microfiche copies from Golden Age Comic Downloads, the only copy I could find.
Source: Golden Age Comics