(T) Under-engine Protection

Hi,

Has anyone else had experience of refitting one of these things?
As mine was missing off my Stilo when I bought it (assuming the usual - a garage just forgot to put it back on!) .... so bought a replacement part from my local fiat dealer for £52 including the fittings.

I offered it up and it fits fine ... 2 bolts at the front where it slips under the front bumper and then 4 screws and washers for each of the 4 corners ... however the central bit has two holes for another 2 bolts however the plastic bits they should screw to are just two rectangular holes... the garage gave me some metal parts that the bolts screw into ... but they don't seem to fit the rectangular holes ..... could they have given me the wrong bits? or are these bits unecessary anyway ... I'm reluctant to hang the undertray on 4 screws...