The abattoir is in breach of the laws regarding traceability if they have lost your pigs. There should always be a vet on duty at every killing, so contact them, plus Trading Standards.
This happened to us with 7 sheep a few years back. They tried to pretend some other carcases were ours - twice - but they clearly were not. Ours were Jacobs, with a totally different conformation to the ones they tried to fob off on us. Also the group were a different make-up of sexes. In the end the vet made them give us carcases of my choice, but even then they were nothing like as tasty as ours would have been and we hated eating them. It eventually emerged that they had exported ours as light lamb to Portugal, so there was no way they could get them back. The vet reported the breach of regs, as was his duty, and the abattoir closed shortly afterwards. Since then the rules have tightened up dramatically, so this kind of 'mislaying' of any animal is absolutely unacceptable. Sounds as if the abattoir are acting as if they have the upper hand, but I think that in fact you do - the least they will face is a hefty fine from Trading Standards.