continuous clean oven
If it had a rough grey interior, it was a catalytic lining. These have a layer that absorbs fatty splatters, and gradually break them down using normal oven heat - no need for a special cleaning cycle. The catalytic layer does become less effective over a few years - some brands offer replaceable catalytic oven wall panels.
They are still "a thing" on some European-made ovens, and Samsung sells a catalytic oven here in Australia too.
I'm not sure if any US brands still offer a catalytic oven, the last Australian oven manufacturer, Electrolux, no longer offers a catalytic version - it is either manual cleaning, or pyrolytic.