You can tell by the Engine Number the first 3 numbers
Firts number year
2nd engine Family 350cc = 55
500cc = 56
Eg 9555**** is a 1999 350cc
Eg 8664**** is a 1988 500cc
by the late 80s ktm whent to Vin Numbers (17 of them) on the head stem the first letter is the Year
1990 = l
91 = m
92 = N
93 = P
94 = R
95 = S
96 = T
97 = V
98 = W
99 = X
2000 = 1
My Brain Bank stops at 1985 I bleave this to be 90% wright ...