Police saw the three apparently experimenting with devices, it was alleged at Caernarfon Crown Court, Gwynedd.
Members of MI5 secretly entered the premises and photographed evidence at about the time the devices were posted in December 1991, Martin Thomas QC, for the prosecution, said.
He said the bombs were contained in video cassette boxes meant for Sir Wyn Roberts, the Welsh Office minister, two senior policemen and a Tory agent. The devices were meant to 'burn, maim, disfigure or disable'.
He added: 'That is what a major part of this case is about - a move from burning homes to burning people, and Welsh people at that.'
Sion Aubrey Roberts, 21, of Llangefni, and David Gareth Davies, 34, of Gwalchmai, both Anglesey, are charged with sending explosive devices likely to endanger life or harm four people.
The four were: Sir Wyn; the North Wales Tory agent, Elwyn Jones; the head of North Wales CID, Detective Chief Superintendent Gwyn Williams, and Detective Inspector Maldwyn Roberts, the man heading the special arson squad investigating the activities of the Sons of Glyndwr Welsh nationalist group.
Mr Roberts, a machine operator, and Mr Davies, unemployed, are also accused of conspiracy to cause explosions.
Mr Roberts is further charged with conspiring with the third accused, Dewi Prysor Williams, 25, of Trawsfynydd, Gwynedd, to cause explosions. Mr Roberts alone is accused of possessing or having under his control explosive substances with intent to damage property or endanger life. All three plead not guilty.
Mr Thomas said the bombs were intercepted at the sorting office in Bangor. A postwoman became suspicious when examining one package and later three others were found. Each contained an explosive mixture.
Mr Thomas said police watched Mr Roberts's flat from a vantage point that gave them a clear view inside. On one occasion Mr Roberts and Mr Davies were seen there. 'There was some activity involving tape, wire or string,' Mr Thomas said, and on three occasions there were powerful flashes from light bulbs.
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