It's not known what year the Volkswagen engine D 1408412 is. The letter D does mean that the engine is a 4 cylinder engine. Earlier engines used one or two letters with numbers.
2004
1967 1500cc
1971
1973 , 48hp , 1600cc
1957
1960 1200CC
That is a model year 1971 through 1973 1600 cc engine.
This would be a model year 1967 Type 2, or bus engine.
to find the year, you need to know the letters prior to the number
1600cc from Model Year 1971 - www.lennysvw.com
This would be model year 1963.
try - bustopia.com