As others have said, that's indeed a horizontal dropout. It looks to be the short version; can't tell if it's stamped or forged from the photo.
The late Sheldon Brown's website has a page with an explanation of the various dropouts and an excellent diagram showing the differences between them. The diagram is found not quite halfway thru this page:
https://www.sheldonbrown.com/no-derailers.html.
For the reasons noted in the other Sheldon Brown page cited above, you really should use an enclosed-cam skewer with that type of dropout.