The majority of the Darlot Petzval lenses which sell on eBay are projection lenses. If they don't have a serial number and don't have a waterhouse slot, chances are they were made to be magic-lantern projection lenses. They could also be very old, pre-1858 portrait lenses, but this is pretty unlikely. From what I have heard, the projection petzval lenses work just fine as portrait lenses. The smaller ones for magic lanterns have the nice focusing rack, while the longer ones were just in barrels. There is not, as far as I know, any difference in design between the portrait and the projection petzvals, but I would imagine that the projection lenses were made cheaper, and perhaps (?) with less quality control.
I once tried to use a 8" f/1.2 (!) lens from an early projection TV as a portrait lens, but the result looked pretty poor on the GG so I just gave up and scraped the project.