Fences
Grey Bandit Wall, Siding, 2x2
While not as sturdy as solid concrete, iron constructions have high structural integrity, requiring fewer support pillars.
Harvest drops
Iron× 17
Taken apart with a harvesting tool (axe, pickaxe, shovel), before perks and tool bonuses.
When it collapses
Falling, it leaves 
Scrap Metal Pile (75% of the time) where it lands.
Where to find (9 POIs)
Minotaur Theater (tier 5), U.S. Army Post #7 (tier 5), ChemTech Distribution Center (tier 4), Don Diego Estate (tier 2), Caravan Carousel (tier 1), Hook, Line, & Sinker (tier 1), The Simmons Shanty (tier 1), John & Sons Construction, Trader Jen.
Points of interest built with this block (any of its variants), the highest tiers first, from the game's prefab files.