One of My friends had a chipper with a Cat/Perkins, a larger engine than on the welders. People in the trade call them "Kitty Cats", they are a new design and don't look like an older Perkins. I don't know what is on the welders.
I don't know about all cats in millers but at least one model is a perkapiller, it's cat yellow with a cat tag and cat filters but has Perkins cast into the block. Identical engine to a Perkins 404d.