Validating the fixing needs of the main SSB did not reveal much. Allowing for the 13.5kg unit a free fall drop of 5 metres resulting in a sudden dead stop (no distance allowed for deceleration) requires an overall fittings strength of 1.7 kilonewtons. The issue is then finding the shear capacity of A4 bolts in kN/mm2.
Stainless steel may have a tensile strength of 500 n/mm2 and an M6 bolt should have , leaving aside 1mm for thread, a total load area of 19mm and therefore tensile strength of 9.5kN. Not the same as shear but the load value indicates a level of comfort spread across 4 bolts.