It can happen. Some years ago we had taken Jacobs to our local abattoir, but the carcasses didn't look to be of a Jacob conformation, but had more rounded gigots. I queried this but only proved I had the wrong sheep by determining their sexes, which didn't match what we had taken in. They swapped the carcasses but they were still wrong. Eventually it emerged that my lovely grass reared, tasty Jacobs had been sent off to Portugal, leaving me no choice but to accept substitutes. I could barely eat the meat, which didn't match what I should have had. As our own vet was in attendance, the abattoir was reported, investigated and soon after closed down, for failing to maintain full traceability.
You should get back weight tickets from the abattoir, which will also have your name on as a cross reference.
Sorry but I can't remember dead weights of our Jacobs. Your more recent weights would be at the bottom end to average of what we would expect from Hebrideans which are a much smaller breed than Jacobs.
Being pragmatic, before you complain you need to be sure of your facts, and decide if there is an alternative to that abattoir before you fall out with them.