Just because it was registered in 1970 doesn't mean to say it can't be a 3...
When I worked in the car industry (in the good old days, when there was only one registrtion per year!!!) if you bought a car in September it was the new model year.... So a car bought in Sept 1970 could actually be a 1971 model year......As the suffixes ran August 1st to July 31st
But looking at the registration it being a H plate, it would suggest it being a 1969.....
Who know's!! Maybe just had loads of mods.........
Suffix Letter Identifier
SUFFIX YEAR OF RELEASE
A 1963
B 1964
C 1965
D 1966
E 1967
F 1967
G 1968
H 1969
J 1970
K 1971
L 1972
M 1973
N 1974
P 1975
R 1976
S 1977
T 1978
V 1979
W 1980
X 1981
Y 1982
Prefix Letter Identifier
PREFIX YEAR OF RELEASE
A 1983
B 1984
C 1985
D 1986
E 1987
F 1988
G 1989
H 1990
J 1991
K 1992
L 1993
M 1994
N 1995
P 1996
R 1997
S 1998
T 1999
V 1999
W 2000
X 2000
Y 2001
Perhaps it was an ex Mod vehicle.......