Not a dumb question, if this sort of stuff interests you, like it interests me. This 686 CS1 was commissioned in 1988 by the US Customs Agency (during the pre-MIM era). The agency had pretty stringent requirements and lore is that these guns were made as well as the first Performance Center guns, even though that division did not exist yet back then.
To paraphrase the more in-depth post below, the rumor of these CS-1's being "better made" and having "DX" accuracy is likely true. Periodically a 686 CS-1 was selected from a production lot and required to demonstrate the durability and accuracy requirement. If it failed to do so it was sent back to S&W and a subsequent example pulled and subjected to the same test. We are told that there were few that failed - but there were a few examples.
As to their rarity, most of these were service weapons owned by the federal government. Approximately 2500 686 CS-1s were released through the Sales Exchange Program before then Attorney General Janet Reno ended that program and ordered federally owned service weapons destroyed rather than sold.
More information can be found here:
https://smith-wessonforum.com/threads/us-customs-service-686-cs-1.231410/