...I wonder what this small hole on this runner was used for? Probably some type of sensor I suppose.

These 911R Magnesium Intake Manifolds are NOT really Magnesium! They are a "perfect" reproduction of the Porsche factory 911R Magnesium intake manifolds,
but cast in Aluminum instead. These manifolds were cast by Tilton Engineering years ago as a request from a "good friend".
The reproduction is so good (finish and all!) that the only way I could tell was by the missing factory casting numbers and the fact that these are heavier.