I've only ever seen two SCO installations in my life, so they must have been living solely on Unix licensing.

They (I am talking the original Santa Cruz Operation now, not Caldera) were quite strong player in their best days. After licensing Xenix (early System III port to 8088) from no less than Microsoft (who in turn licensed the SysIII source from AT&T) around 1980 SCO nursed it until more capable processors came out, and Xenix/286 and Xenix/386 in various versions and flavours were quite popular. A bit later came SCO Unix (later called SCO OpenServer), a relatively good, 386+ port of SysVr3, which is virtually unchanged to this day. I saw hundreds of them. The main competitor (there were many) was Interactive Systems Corporation product, also SysVr3 (significantly better than SCO, IMHO), and the competition was quite brisk. A month or two after ISC launched SysVr4 as one of AT&T's principal publishers, Kodak sold it to Sun, which ruined then discontinued it in short order. Soon AT&T sold its Unix System Laboratories (with Unix and excellent transaction monitor Tuxedo, now owned by BEA) to Novell, which didn't quite know what to do with it, but continued to develop a version of SysVr4 half-heartedly. It became UnixWare. Then SCO bought Unix operation from Novell (Unix name went to X/Open, Tuxedo to BEA), had mostly failed joint development effort with HP, focused on Tarantella (whatever that might be) and sold Unix to Caldera, which itself was Linux-focused startup by ex-Novell people.

My company was a very early adopter of Unix on Intel for business applications, so I lived through all of this. Fascinating history.

Er, what I was actually trying to say, there are many SCO OpenServer and UnixWare installations.