The S&W PN# for the 4586TSW's firing pin retainer is #266920000 ($43.01 old catalog price, new). The catalog also lists a firing pin retainer spacer, #233880000. My 4586 (non-TSW) doesn't have one & I can't say if it's needed on the TSWs or just an optional "fit" part.
S&W may have it. Numrich list it as available:
Firing Pin Retainer, Stainless Gun Parts | 347300 | Numrich Gun Parts
Just one other thought on the TDA's decocker/blocker. I personally like having the decocker/blocker down whenever I'm loading/unloading my 3rd Gen, & not in a safe environment, e.g. range. It's just comforting to know you've greatly eliminated the likelihood of an accidental discharge by physically blocking the firing pin from the hammer.
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