Yes, that's my understanding too. A metal (probably lead) disc for extra weight.

And in other cases, metal filings (sometimes lead (?), but brass usually, I think) added to the "clay" mixture throughout the chips. Also to add extra weight.

When the filings were reduced to a powder (that is, lead powder) and added to the chip, it caused a ruckus with the health authorities, as it was a potential health hazard, like lead paint, etc. Not sure how this was resolved, other than the lead powder was discontinued. There may be (?) some older chips out there (in our collections) with this issue, though not a widespread problem.

(I know you're aware of all this, but expanding on the issue for others who may not be.)