What was the original make of tires that were on the 1976 Porsche 914 2.0 when the car was sold new?

Don't know and it doesn't matter. You can't buy them anymore. Best guess is that they were early steel belted radials or perhaps nylon belts that thumped until they warmed up. Try http://www.cokertire.com/ for period specific tires made to today's standards. I've been buying from Corky since '85 when I needed square tread tires for my sidecar rig.

I use Vredestein Sprint Classic on my '76 914, tire (tyre) made in the Netherlands, to maintain the period look