1972. Good poll, there always seem to be a lot of 1972 vans around on tlb, will be interesting to see how many.
march 6th 1978 is the build date on the m plate and the vw birth certificate, however the dvla put 1.1.78 on the v5, i rang them to complain and was told they only care what date it is exactly if it's tax free or close to being a different registration letter!