A fullblood maine is one that is in the Canadian registration with typically uninterrupted parentage verified with DNA markers. typically, this goes back to original import bulls, cows, pregnant cows, .
most of the "fullblood" Maine's only registered in the American registry have not been parentage verified, hence you have national champion polled bulls out of horned parents. this also renders any epd's derived from unverified parentage, unsuitable for use.
Polling from horned parents could have been a mutation, but since no parentage has been confirmed on all animals, their parentage can not be confirmed. There were two separate verified mutations in the Canadian registry from which all polled fullbloods come from.
here is cunia's page in the canadian registry.
note both the old system, blood typing, and the new system, DNA markers.
an animal without uninterrupted dna verification is a purebred