In inherited this watch from my grandpa I see its a capped one but usually on the cap the back is from steel. Anyone idea what this one is
I think it's either gold filled, gold plated or maybe even solid gold (someone may be able to tell from your photos but I can't). The marking inside will reveal all. Take it to a watchmaker (it's a front loader) to have the movement removed so you can confirm. Edit: There is writing on the back, what does it say?