What 1st gen Mac Pro do I have

You seem to have got yourself one of the ultra rare MacPros that due to a manufacturing error were built sideways.

Looks like a MacPro 3.1 2008 model.

Start the Mac up, then click on the Apple top left of the screen, click on About This Mac, and a small panel will open informing

you which Mac Pro it is.

That is definitely the early series, 2006, 2007 or 2008.

If it won't power up, there was often a label near the graphics card slot with original configuration information tersely printed.

(Please don't post your serial number on an public forum.)

The label I am hoping you can find is on the small horizontal surface directly under the graphics card slot and above the rear fan.

DO NOT post the serial number. Just post the rest of the information such as Processor Speed.

I might revise that to either a 1.1 or 2.1 the 'aluminium' around the RAM boards is different in the 3.1, instead of one large aperture

there are two apertures round each RAM board.

That model can run El Captain with Apple's blessing.