#2 is the brace that bolts to the compressor front (via #16) and #3 is the adjustment bolt to loosen to help slide it along the groove when adjusting the compressor belt tightness. Bolts #7, #16 and #3 has to be loosened so you can adjust belt tension, then when tight enough, tighten #3, #16, then #7. Boom.
#3 is M10 x 1.5 x 25. 10.9 hardness.
#3 is M10 x 1.5 x 25. 10.9 hardness.