Name / Adapter for Vintage AC Plug
After some further research I've found out this connector is called a PH-163.

You can find them here: https://marvac.com/products/vintage-switchcraft-lac3gd-ph-163-oval-type-hp-connector-3-pin-round-ac3g

Adapters look like this: https://www.amazon.com/Vintage-Oval-Hewlett-Packard-Power/dp/B011CAFIGC