Why yes, this was a filmed version of the stage play, you're correct! As I'm trawling through a lot of these old films, I'm surprised at just how many of them were stage productions first – I guess it makes sense given traditional screenwriting was predominantly embryonic at this point. As much as I found it a little too "stagey", one gets the sense that audiences of the time would have found this utterly mesmerising. Lugosi commands the screen.