The caseback is stainless steel, which is common with watches that are gold-filled as this one, or gold-plated as happened later on. Gold-filling involves bonding a layer of gold sheet to the case, so is typically much thicker than electro-plating and less likely to wear, but it is only the solid gold watches that get gold casebacks. The seller does state in the ad that the watch is gold-filled.

Good price even so though, though I don't know quite what was going on with the postage cost!