I had a mid-1970s MKM with the wrapover stays, that I sold about 15 years ago. I agree that the Portacatena dropout dates it circa 1978-1983. 1978 was also around the time they added the stay cap engraving and top tube cable tunnels. So, it appears to be from after the period of Mason's and Metcalfe's involvement. Fortunately, MKM frames from the early half of the Portacatena era can be narrowed down based on the serial number.

1978 serial numbers ~ 4800 to 5700
1979 serial numbers ~ 5700 to 6600
1980 serial numbers ~ 6600 to 6805