Look like you have hydraulic tensioner from the picture. The mechanical tensioner will have a 15mm hex on the housing. You have hydraulic tensioner if you dont see that bolt.
BMW E53 X5 M54 3.0L > ES#2187565 Mechanical Belt Tensioner Assembly - 11281427252
I have mechanical ones on both my BMWs so I can't comment on your empty bolt thread. How many miles on your X5 when the belt snapped?

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00 E39 DINAN S1 //M5 (82k miles)
06 E53 DINAN 4.8iS, born 2/18/2006 (126k miles) http://www.xoutpost.com/bmw-sav-foru...s-my-dslr.html
90 Straman Z32 TwinTurbo Convertible, (1 of 44 ever made) 5 speed, 444rwhp/451rwtq
01 360 Novitec Spider F1 (26k miles)