Court sets bondin bombi% ' ;The son of a Casa Grandeminer, arrested by postal officials in Tucson in connection with the planting of a packaged bomb that exploded in his father’s hands, .was arraigned yesterday and ordered held in lieu of $25,000 bond.William L. Webb, 20, was arraigned in Phoenix before U.S. Magistrate Richard Gormley on a federal charge of possession of firearms not registered under the National Firearms and TransportationAct. • ■ •' !.•; ? ,'ff .Webb was arrested Tuesday in Tucson in connection withug casethe bombing last Thursday, which seriously injured- his father, Newell Webb, 41, a lft-year employe of American Smelting Refining Co.iThe elder. Webb was listed today in serious condition in a Phoenix hospital.. Both forearms were amputated following the blast.Postal inspectors were able to' reconstruct the package that contained the explosive and found several fingerprints that matched the younger Webb's, investigators said.A • motive for the incident has not yet been established.