All GM part numbers are 8 digits and are usually stamped/embossed on the part itself.
If you want a genuine GM catalytic converter it's part of the front pipe assembly and is exchange only.
Astra G - Z22SE [L61] Engine (Hatch,Saloon,Coupé,Estate,Van,Monocab) (F07,F08,F35,F48,F69,F70,F75):
Part # 55558954 - Pipe Assy,Exhaust,Front, with Catalyst; New Exchange (Core Return No. 9117104)
Brand new genuine ones appear to be very rare, a Vauxhall dealer may have one but I'd imagine it would be expensive.
Hope that helps