[HarryT]

Yes, that's right. "Murder is the Mews" is an expansion of the earlier short story "The Market Basing Mystery", and "The Incredible Theft" of "The Submarine Plans". Both the originals are published in the anthology "Poirot's Early Cases". It's worth noting, however, that the novella versions in "Murder in the Mews" were the original book publications of the stories; it wasn't until many years later that the shorter versions, which had originally been published in magazine, were published in book format.