I guess the bridge may be original, I was just going off what I'd been told. If AJR documented it before the film set was built then I imagine that proves the case that it is original.
As for finding it...
Head up the narrow hill opposite Pete's Eats in Llanberis (Pete's is on a crossroad, head uphill away from the lake), steep hill, levels out as you leave the village then gets narrower. Continue past the rear of the Siemens (was EuroDPC) factory. Road steepens again and the widens with fences on both sides. You're looking for a short straight stretch that goes between two pits - tall fences on each side. Distance from Llanberis probably 2/3 mile. Bridge is at the bottom of the higher pit (on your left if coming from Llanberis). You can park a bit lower down and climb over the fence near the rusty gate.
SRT needed to get down into the pit.
This road continues to Cwm y Glo, or turn back and get a Big Jim in Pete's :)
Hope those rambling directions made sense!
my orders are to sit here and watch the world go by