Identify Vintage Power Cord for 1980s Organ
I'm reasonably certain you need an IEC 60320 C9 plug to mate with that connector.

Wikipedia has this to say: "This kind of coupler is used by Roland Corporation for a couple of synthesizer and drum computer models (for example: TR-909) and by Revox for many older models of their HiFi equipment (for example, A76, A77, A78, B77, B225). Was also used heavily on Marantz Hifi equipment in the 1980s and 1990s." (source)