The catalogue reports that analysis of the Abbiate Guazzone pieces found a copper-tin-lead composition and considered it very probably to be material from a heavily worn bearing packing bed.
“It is very probable that the fragments presented to us for examination come from a packing bed of a bearing that has had very heavy wear.”
Source: Preliminary Catalogue Of Analysis of Alleged "Fragments" Reportedly Associated With Sightings Of Unidentified Aerial Phenomena
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