The war-time relic had been in the shop for “several weeks” after being handed in inside a black bag filled with children’s gifts.
The shop, bustling with seven shoppers and six volunteers, and several others surrounding it were evacuated by officers after the explosive was found on February 25.
Sales assistant Nadine Maddan was first to spot the suspect item, and promptly told her manager Sue Humphreys who dialled 999.
Ms Humphreys told the South Wales Evening Post: "Nadine showed me what was inside. We did some research online and it looked to me like a real grenade.
Exhibition marks Blitz and Dresden bombings
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"The pin was still intact. We didn't panic we just dealt with it.
"I've never seen one in real life. It felt heavy that's why we thought it was real.
"I hope it's not live it can't be as it has been thrown about in the bag when we've been moving it around. The bag has been in the shop for several weeks."
A Gwent Police spokeswoman said: "Specialist trained personnel from the Army Explosive Ordnance Disposal Unit have now assessed the device, believed to be a World War II grenade.
"It has been deemed as low risk and has been safely removed."
Police removed the cordon later that day.
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