@indoissetep answered your question:And while we’re at the unanswered questions in TFA...
I just thought that was the same hand, but the synth-skin had been lost. I have this idea that it was ripped out or maybe even melted in some dramatic event and Luke never bothered to replace it.
thethumbthief answered:
Well I’d say the synth skin wore down over the decades. And upon realising that the hand maintained its function without the skin (and also gifted Luke a serious badass image) he didn’t bother to maintain the skin.
These makes as much sense as any I’ve heard.
But it sort of leaves me wondering how Luke feels about it when he looks at it, considering how his father ended up more machine than man at the end. Maybe he keep it uncovered as a reminder of the path Anakin headed down and not to make the same mistakes he did or...?