Info: Date a TY175 or TY250

See Trail & Trials UK blog post. They have details on chassis numbers to work it out.

Chassis No's starting 525-0xxxxx = 1975 TY175B
Chassis No's starting 525-1xxxxx = 1976 TY175C
Chassis No's starting 525-2xxxxx = 1977 onwards, TY175D / E / F / G

Also, the model years for Yamaha's run as follows (These are model years, not necessarily year of manufacture, which may be a year earluer):

1974 = A     1975 = B     1976 = C     1977 = D     1978 = E
1979 = F     1980 = G     1981 = H     1982 = J      1983 = K
1984 = L     1985 = N     1986 = S     1987 = T      1988 = U

For later dates (and Suzuki dates), see cyclepedia.com.