Firstfives.org

One costs $95, the other also says OEM direct fit, ten bucks less, with a note: 2.5 bar.

I give up. Looking in the little hole(s), I don't see any bars to count.

And, I'm assuming fuel pressure regulators are the same for a '79 635 euro and e12's, as they look the same with matching part numbers.

Probably the same. RealOEM is your friend. Bar is of course a measurement of pressure, nearly, but not quite equal to one atmosphere.