I doubt there is scientific journal article on exactly vintage slow cookers in the United States but there are numerous on ceramic glazed pottery. Simply, the standards for lead allowance in consumer goods in the US have been abysmal. Here are some generic articles on lead glazed ceramics leaching. here here and this quote from consumer reports on lead testing vintage ceramics. This includes vintage and older ceramics made before 1992, when the FDA instituted its current lead guidelines for food-safe ceramics; imported ceramics or those you bought abroad (where safety standards may be more lax); or ceramics made by hobbyist ceramicists if you’re unsure they comply with safety standards by using lead-free materials, or whether they use quality kilns that run at the high temperatures necessary to properly fuse the materials.

This investigative reporter Bill Gephardt from KUTV news did a report on exactly this, slow cookers, in 2004. Here is his twitter @gephardtgetsit but not sure if he's still alive. He seems to have website now called Gephardtdaily.com. @kslgephardt on twitter is probably family relation. I cannot find his broadcast since it's before internet news broadcasts were a thing but this article outlines his investigation and the scientific results. He even comments to author at end of article about the attempts to pass new laws related to lead in Congress. I'm sure he can be contacted if he is alive. The article author states she contacted Crock pot brand and they said yes they contain lead (but below then governments threshold in 2004). That's how they used to get around it. These levels have been amended numerous times, in the 70s and 80s their allowable lead level was much worse.

My daughter was lead poisoned in 2016. I have attended lead poisoning symposiums in Washington DC and listened to some of the top lead specialists speak on this topic. I am by no means an authority but believe them when they strongly recommend not using vintage cooking/bakeware/dishes for consumption. It was even investigated by health department when they came to my house. But gamble with it if you want. Though effects on adults still serious. Just don't gamble with a child's life.