I'm confused. You say the mystery city is northwest of Milan, as is Legnano, but closer. Yet you say that it is as close as Pavia. My map says Pavia 43 km from Milan, and Legnano is 29 km. So is the mystery city closer than Legnano, or closer than Pavia, which could make it further than Legnano.
How about ALFA (before Alfa Romeo) at Arese, 17 km northwest of Milan?