Cactus Identification?

[Cosmic Spore]

This cactus was my great-grandmother's cactus; I'd like to know what it is.

It has milky sap, mostly 3 ribbed, but has 1 column base near the soil with 4 ribs.

Has spines that are paired, 2-at-a-time.

 

[Animistic]

I'm not a master cacti identifier by any means but it looks a lot like some kind of Hylocereus (dragonfruit) to me but like I said I'm no master at this.

A.