6 pence error cup coin ?

i have just found a 6 pence 1956 coin and it looks like a cup from both sides and the edge is smooth all around , also the weight is the same as a original 6 pence . as the diameter is the same .
So is this a error coin ?

https://en.numista.com/catalogue/pieces576.html

Modifications done afterwards for jewellery or other purpose.

Verweis : "Frenchlover"​Modifications done afterwards for jewellery or other purpose.

​indeed
https://www.etsy.com/market/1956_jewelry

Thanks guys for replying !

these coins with necklaces i love them very much ! these are fine art !

so how much can i sell it for as it is on ebay ?

also i have another coin like this one , a 1 penny 1965 , is this a coin for necklace too ?