In the pictures of a very large Rivendell moc of some years ago I came across a fence-like piece(I have encircled one of them). I have tried to identify it myself by browsing through the parts catalog in Bricklink but wasn't able to find it. Does anyone know which part this is?
They are custom parts from altBricks.
From their interview with Brothers-Brick a few years ago:
TBB: Were there any structures you just couldn’t find a way to recreate purely from LEGO bricks?
Alice: I know the purists will cringe, but I used a few pieces made by altBricks because they fit so perfectly with the elegant, flowing style of the Elven architecture. The panel piece is one of my favorite pieces in this model since you can can apply it in so many different settings: tower-top decorations, windows, and balcony railing to name a few. I also used their 1×2 column since I like the fine texture of the fluting and the back has a nice arched opening as well.
David: In the bridge section, the altBricks element was key, since tan telescopes were not available and I really needed to project that sweeping flowy look. I tried to remain as purist as possible, but in a build like this we needed that part to really get the look we wanted.
This is also confirmed on Alice's Flicker photostream where she captions a few of the shots with:
Elven 2x2 panels by altbricks.com