https://www.spymuseum.org/press/press-archive/2019-press-rel....
It had a tiny needle that delivered saxitoxin and/or botulinum toxin to the agent, giving them a fast and very painful death.
AFAICT, there's no public record of how many of these were given out and how many people used them. But we do know of one famous case where someone didn't use it — Gary Powers. He had one of these silver dollars and he refused to use it. He chose capture and torture instead.
This is what his suicide dollar probably looked like, https://images.seattletimes.com/wp-content/uploads/2017/09/a...
For me, personally, there's something very funny about the CIA turning one of the country's most recognisable symbols, the dollar, into a suicide pact.