I recently had a serpentine belt break on my 3.0L X5, and was able to put on a spare and drive home. I went to order new pulleys and belt and was informed that there are two types, mechanical and hydraulic (with different pulleys). Is there an easy way to tell the difference without removing all the housings and clutch fan again so i can order the the correct pulleys?

Additionally when i was snapping photos to see if i could get a view of the hydraulic piston, I noticed what appeared to be fairly clean bolt threads into the engine behind the deflector pulley. I attached photos below. Did some bolt back out causing serpentine to break, or is this just unused bolt threads?

Attached Images