What kind of outlet is this?
Let's see now.
A vertical small prong (hot) and sideways large prong (neutral) would signify 120V / 20A. On the receptacle the sideways opening gets a vertical aspect for a combined T shape so it can accept a common 120V / 15A plug as well.
On the other hand the small opening being sideways while the large is vertical signifys a 208-240V / 20A outlet and the receptacle's vertical side is also given a horizontal aspect for a combined T shape so it can accept the rather rare 208-240V / 15A plug with two flat prongs sideways.
The outlet shown is obviously to some ancient standard. Looks like it will accept anything...except a ground pin. LOL.