Yes. ATX is indeed a few years newer than this. I actually found this page: http://www.smps.us/power-connectors.html and it looks like it could be the same thing as "DELL PENTIUM II and III" connector mentioned on that page. At least the non-connected pin 19 is non-connected and I can measure …

I've always wanted to build an original P5 based 1993 Pentium system. I know it is not the most practical retro computer. But I want to have some fun with the FDIV bug and refresh some early 90's games. After some effort, I finally sourced a suitable board. I got it cheaply off eBay and it is new …