Agree, looks to be a Super Special, which has the integrated HS versus the later Nulli Secundus. I had a 1952 Nulli, perhaps my favorite Latin lug pattern. The Hetchins website link is a great source of info, inquires there may provide more detail on the HS specifications and the frame itself as there is a registry. Not sure if the website is maintained routinely still.

Nice project. Perhaps it was a Road/Path or TT frame designed for fixed or single speed with that dropout spread. In this era, frames were custom so who knows the whims of the original customer.