I'm not a vet, but to me an intramuscular painkiller is like giving general pain relief to reduce pain, much as you would for a headache; whereas a local anaesthetic is to totally block the pain in a specific area. Our vet has always used a local and our animals haven't felt a thing. Recently I get them to administer Metacam (painkiller) afterwards for when the local has worn off as I read that this reduces the stress to the animal as the feeling comes back.
Sounds to me that it's no wonder the testicles disintegrated as the poor calves must have been in considerable pain, and no doubt struggled violently.