I love digging up old threads but think I've got a worthy contribution to this. All be it not specific to Commers.
First things first, the Build Date is what dictates whether a vehicle is classed as a historic vehicle and is therefore exempt from paying for road tax. This build date has to be pre 01 January 1973. ie Built in 1972 or before.
The registration date is what is on the log book.
A car has to be built before it is registered. Therefore if the registration date is after 01 January 1973 it does not automatically mean it isn't going to be classified as a historic vehicle.
In fact, from memory I seem to recall that teh DVLA will allow cars that were registered early in 1973 to be reclassified as historic as it is assumed/expected that they were built before 1 Jan 1973. Very decent of them. From memory it is anything registered in the first two, or possibly three weeks of 1973. Practical Classics had a big article on it some time ago.
For example;
I have a Manta that was registered in July 1973, but was built pre August 1972. I have been able to prove this via information from the Opel factory and the Chassis numbers. Therefore my Manta is classified as historic.
Conversely a friend of mine has a Manta that was registered in March 1973, but was built in Feb 1973. Therefore his is not classified as historic and has to pay tax. (He wasn't amused either, I can tell you

)
Hope this example helps.
Furthermore though, when I tried reclassifying my Manta I had further problems.
The DVLA office in Lincoln wanted to issue my Manta with a K-registration instead of the L-reg it's had since new.
The reason for this? Because I couldn't prove an exact build date they wanted to default it automatically to 01 January 1972.
Apparently this is the usual practice for cars where an exact build day cannot be determined. So they do not appear newer than they are and therefore sell for more money.
Obviously I wasn't happy with this as the 30-something years of history would be worthless and it would affect the cars resale value. But no they were determined.
Eventually, a little old lady who's been there since the dawn of time happened to overhear what was going on and quoted a rule that even the manager and so called specialists there didn't know, stating that historic vehicles are exempt from this first law. In the main because its mileage & condition much more than a couple of months on the age that affects a historic cars value.
So, a word of warning, if this happens to you, stand your ground!