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.