On mine (power plant HP), the nuts were captured by the winch itself, and the bolts went up from the bottom of the plate which is why they were square. The other two look like the fairlead mount bolts. The plastic caps in mine were spare to cover access holes under the cover (maybe how the motor housing comes apart?). I just kept them for the future. No idea what the others are for. There should be some instructions. If not, I think Warn keeps them online in PDF's too.