Hi - My opinion is that this guitar was made in 1983 (Going from the stamp on the neck block. I've got a number of Matsuoka guitars and the lowest number I have stamped on the neck block is 74 - I also have a 77, 78, 81 and 83. I have a couple which aren't stamped on the neck block though - one of which has ano: 1973 on the label in the sound hole. Nearly all the Matsuokas i've seen have been rosewood back/sides apart from the M30F which is maple. Yours certainly does look like mahogany back/sides - unusual! It looks like the fret board is ebony which to me suggests an M30 or higher rather than an M20. (That's speculation on my part to some degree though!). Hope that helps!
More details on Matsuoka guitars here:
http://www.kaitunes.com/guitars/acou..._matsuoka.html