We think these rather interesting fruit are Maguey, which is the fruit from the Agave, the big spikey-leaved plant from which tequila and mescal are also made. We bought a couple, and when split they have a very large shiny seed, and soft sweet flesh which tastes rather like some kind of pudding. This vendor rapidly emptied his barrow at the Tianguis, the massive open-air market on tuesdays here in San Miguel.