Hi and welcome.
Without pictures it's hard to make a guess but Matsumoku made a guitar with the same model number under the Westone label:
http://www.westoneguitars.net/guitars/w-40-acoustic/Does yours look anything like that one?
A 6 digit number usually means the first digit is the year which would possibly make it 1984, but that's not definite, it could be as old as 1974, that's why pictures would help.
7 digit numbers are more accurate but they came into use sometime late in 1984 with the next 2 digits showing the month.