The second box down with number ending 400 is your VIN to added to the V62 form.
Bare in mind that it was probably on the road a long time ago and hasn't been registered for ages. The original Registration plate if interesting could have been transferred. DVLA's records have only recently been computerised any old records are likely left in a dusty archive.
Facts the DVLA are interested in when you attempt to register a vehicle are:-
1) Is it a real vehicle or something you are attempting to pass off as something else. To prove this you should send a couple of good photographs to prove the vehicle actually exists and it is what you are claiming it to be.
2) Does it have a set emissions CO2 level. You need to prove the age of the machine puts it before CO2 levels where introduced.
3) What taxation value will apply to it. You need to prove it is old enough to be tax exempt (nil rated historic) or otherwise.
In the case of tractors or military vehicles I have registered it is sometimes helpful to involve an owners club or DVLA recognised body that will vouch for the provenance of the vehicle. This makes it simpler to register. The other method was that sometimes you were asked to take a vehicle along to a DVLA office or have someone come out to inspect it. Have you tried contacting JCB for a proof of age of the vehicle?
What responses have you had from the DVLA on the three occasions you have dealt with them? Surely they have given you an indication of reasons for refusal. Is there not a local DVLA office you can call in to speak to someone knowledgable in person? Have you paid the £25 fee each time?
http://www.nationalnumbers.co.uk/dvla-guide/dvla-local-offices-42.htm
