Unfortunately, there were many, many bikes manufactured in Italy that are very similar to this frame that it's kind of tough to say who may have made it.
Internal cable routing wasn't uncommon when this frame was made but it does look more like a mod than original work.
On this mystery frame, I would expect to see internal cabling finished off with a 'grommet' like this one (from an older, 1951 frame):





Also, it looks like both orange and purple are respray colors. Is there a light blue I see inside the seat lug? Maybe that's a remnant of the original?

Don't know if this will help or not, but the serial number format and position (and the almost home-made style) reminds me of numbers on some Bartali badged frames. Here are some examples I've come across:




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There were Bartali frames with bullet seat stay caps and internal cable routing as well.

Judging by the age of the mystery bike here (and if this was a Bartali badged frame), it would be from the Fratelli Margherita period rather than the Santamaria (which ended in mid-50s), although Bartali could have contracted other builders later on as well.