Let's just say the evidence is not conclusive that it is a Miyata. Falk was generally used on Italian brands, so the first thing I'd be doing is checking the threading on the bottom bracket. It should be marked on the fixed (drive side) cup. Assuming the bottom bracket is also SR, an Italian cup will be marked 36 x 24T. A Japanese frame would use an English cup, would be unmarked or marked 1.37 x 24T. Given the apparent era, French threads are also a possibility and they should be marked 35 x P1. But those look like Suntour dropouts, so I'm guessing it is English threaded, in which case, a Japanese brand is most likely and I'm at a loss to explain the Falk tubing.