Thanks for the confirmation Fastbolt! Sounds like that means that other pistols share this plastic button besides the CS series of gun. Since Brownells doesn't list any parts specifically for the CS45 I'll have to pick an alternate gun to shop under. After searching the S&W parts list for 239090000 (part number listed under my CS45) I find matches in M410 / M457 / M908 / M909 / M910 which are all covered on their website.

I'll definitely put one of the buttons on my next parts order. I intend to have the aluminum frame re-anodized in the not too distant future so it's going to have to come apart.