this decoder says end Feb 74 beginning March 74 for mine, but it does say not 100% accurate, I feel your pain too
I thought the same thing. As it's build date and not registered date I guess the owness is on the owner to prove their entitlement.
But that is out-of-date now, as they haven't bothered updating their leaflets after the closure of all the DVLA local offices. However instead of taxing for the first time at a DVLA local office, it appears that this work can now be done by post offices. Thankfully mine seems to be reasonably clued up on these things, some others may not be.
This is all very confusing. Does it mean that a build date in January 1976 will not be tax free until April 2017