In the MOTS study I found this: "Treatment capsules, referred to as “MSDK” were formulated using 5 mg melatonin, 450 mg strontium (citrate), 2000 IU vitamin D3 and 60μg vitamin K2, divided into two capsules. Identical placebo capsules matching in size, shape and color contained plant fiber. Study capsules were formulated and manufactured according to the principal investigators' specifications and supplied by Pure Encapsulations, Inc. (Sudbury, MA, USA)."
Personally I am unconvinced that increases in BMD via strontium equate to stronger healthier bones. It supposedly makes DXA scans report higher density than you actually have. Plus, just because strontium and citrate are natural doesn't mean that high doses are safe. Many trace minerals are damaging in high doses. 450 mg of strontium is a roughly 100 times higher dose than you might get from your diet. I'm unconvinced but it seems there is no definitive answer about strontium at present.