M = Merida or Miki, the manufacturer
9 = 1969, 1979. 1989, etc
K = 11th fortnight or 11th month
6445 is the sequential frame number for the fortnight or month.

Of course the decade for the year can usually be determined by the components and general frame characteristics. Miki was a Japanese manufacturer and Merida was a Taiwanese manufacturer, both of which used a similar serial number format. There should be a country of origin sticker on the frame but given the era, Taiwan is more likely.

If you search the forums you should find a serial number guide for known vintage bicycle brands that were manufactured in Asia. The brand is cross-referenced to known manufacturers and their serial number formats. It used be a C&V Stickee but the Mods moved it.