Not just custom - Seiko used to (still does maybe) provide manufacturers with bare movements, even with the company name on it. That means you'll find "no name"

Not just custom - Seiko used to (still does maybe) provide manufacturers with bare movements, even with the company name on it. That means you'll find "no name" watches from the 60s onward with Seiko movements. That makes servicing, repairing and replacing them much easier and cheaper. Same with ETA.