here is a small Info about SA-404,SAI-504 and SAI-307 Fighter Projects.
Should be SAI-Ambrosini
S.404,
S.504, and
S.307 projects
Rough translation with a bit of editing:
Even the prolific Ing. Sergio Stefanutti, designer of the SAI-Ambrosini S.207 family and of the promising S.404/504 projects with Zeta engine, offered an unidentified "Aeroplano da caccia stratosferico". That 'stratospheric fighter' was possibly later designated S.307. With a 1,000 hp Zeta RC.110 engine, this fighter would have had similar performance to the Serie 5. However, the Stefanutti design would remain on paper. Incidentally, the preliminary projects of the S.404 and S.405 family was the result of work by Ing. Lelio Rossi - at that time, one of the managers of SAI-Ambrosini's technical office.
Speaking of the S.404 and S.405 family, it should be remembered that these would have been Series 4 fighters, since they would be powered by the Isotta-Fraschini Zeta engine.
These projects, of which drawings dated 14 August 1942 are found, aimed at solutions for simplifying construction and reducing costs. There were to be three different configurations: S.404 with a 10.80 m wingspan, S.404A with 11.40 m span, and S.504 with 11.80 m wings. The program was carried out with a low profile, since the company was meant to be mainly engaged with the S.207 and S.403. In any case, the IF Zeta was never actually available. With the amistice, further development was suspended. [...]
Ing. Lelio Rossie (already mentioned with regard to the SAI-Ambrosini S.404 and S.405), went on to the technical office of Caproni in Taliedo (we do not know if that was in an official capacity or as an external consultant). In the company's documentation there is reference to his study of a jet fighter "with a Jumo 004 engine; in reality, the project seems to be the R.105, dated January 9, 1945, for which the R.2000 turboprop was considered. Rossi was killed on April 27, 1945 for political reasons and, as a result, there was no way to continue his projects.