I am sure that radio is at least 25-30 years old? How could I tell when a radio was manufactured? It would be interesting to know...
The MX-385 radio in my former 152 has a serial number on the back. I'm sure that if you contact TKM and provide the serial number of your MX-300 they can tell you when it was manufactured. Also, my radio has yellow tage dated 7/1/02, so it must've been made prior to 2002.