I have schematics as early as 1937, but the odds that the manuals stayed with the machine are pretty low. (Usually these machines were owned by somebody who had a bunch, and I suspect the manuals tended to get collected centrally.) You can generally buy manuals on eBay, and some are being reprinted by people who own the original IP.
The internet has a lot of scans of wildly varying quality. The AI industry's hunger for data means there has been a lot of progress in OCR and data extraction, so I have notions of taking something like PaddleOCR and trying to turn the scans into a cohesive reference site that will work well on phones, etc. But I'm not sure when I will get to it.
If people out there are interested in working on a project like that, let me know. Email me at william@theflip.museum.
I know very little about pinball machines, out of curiosity, do some come with any amount of schematics or diagrams?