It sounds like somebody bought some Yamaha stickers or a tank or something, and stuck them on your bike. There is no Yamaha KS250, and if there was, it would be 250cc.
Try finding the serial number on the bike, on most bikes it's usually stamped into the steering head behind where a headlight would go, and google it.
BTW, the tenth digit in a Yamaha's serial number (vin) stands for the year it was made. I found the following chart on the web.
A: 1980
B: 1981
C: 1982
D: 1983
E: 1984
F: 1985
G: 1986
H: 1987
J: 1988
K: 1989
L: 1990
M: 1991
N: 1992
P: 1993
R: 1994
S: 1995
T: 1996
V: 1997
W: 1998
X: 1999
Y: 2000
1: 2001
2: 2002
3: 2003
4: 2004
5: 2005
6: 2006
7: 2007
8: 2008
9: 2009
A: 2010