Rolex Bubbleback Identification

Request the help of forum once again.

I was considering a Rolex Bubbleback 3015 but what is stumping me is that even though caseback is steel, inside caseback is pink gold with 18 K stamped.

What is the issue here? Is it genuine? Vant find a similar piece online.

Sorry, dont have outside pic of case back but it is steel!! (Or is it polished?)

Thanks