It could be one of the early HP Vectra series, based what could be the mobo part number: D2360 - seen on label & copper screen.
(I have a similar model in that series, but with the CPU & memory on a plugin board, so it is not identical. I did find a site with some pictures of it some time ago; I'll see if I can dig it up again. One thing to note, is that the PSU connections were not fully populated on my PSU cables; this could possibly differ between models.)
Edit: I did not find the pictures I was looking for, but there is this old service manual that *may* contain what is needed to power the board (registration needed, pay service, but free for a 30days tryout they write):
https://www.scribd.com/doc/151243045/1995-HP- … August-1995-pdf