It looks like a manual can be found at
https://www.scribd.com/document/7036...ruction-Manual I don't know how scribd works and haven't been able to download from there. From my understanding White Facebook Group has a PDF of the White 628 owners manual. I don't do Facebook. I have a pdf manual for the White 628 that I got six years ago from singer.com (which no longer has vintage or White manuals).
From the research I've done about this machine, it looks like it was sold in 1960. It takes a high shank foot. There is a 15 year old video about this machine
https://www.youtube.com/watch?v=3ACsVTK-JkM which states it is an all metal machine. It also shows sewing with it on various thickness of fabric.
I wasn't able to find an adjuster's manual, but it looks like a fairly easy machine to maintain. The oiling diagram is on the last page of the manual. I can't tell if the top arm comes off easily, as it shows the oiling holes on the top of the top. I would think that the top should come off so that cleaning, adjusting and oiling could be done.