Seeking the name of a lost 50s/60s font from Prince's Purple Rain

I'm searching for the name of a font that was used in 1984 on Prince's Purple Rain album. I think the font likely dates back to the 1950s or 1960s.

The designer of the album no longer remembers the name of the font and is unable to look it up. More recently, a font called "Morgus the Magnificent" (available at https://www.dafont.com/morgus-the-magnific.font) was created based on this original font. I've been in contact with the designer, who said he took if from an old advertisement but never had the complete font, so also doesn't know the original name. 

I've spent considerable time researching and consulting various type specimen books, but I haven't found an exact match. I am specifically looking for the name — I don't need a working version. Any insights or leads would be greatly appreciated. Thank you!