Is My 1961 Three Pence A Mint Error

The Royal Mint excecutes orders for circulating coinage of many other countries.
This coin is most probably struck from a round bronze planchet intended for one of it's customer countries.

I note that this coin appears to be a lot rounder than a normal brass threepenny bit. A normal threpenny planchet before striking, would be dodecagonal.

Thanks for your input guys. I think I will send it to The Royal Mint to see if they can explain what went wrong

It was a period slightly later than 1961 that weird error things were happening to British coinage. I suspect that your coin is a genuine accidental error.

1967 is a time when errors were far more common.

1967 was a time when the Mint was asked to execute the change to a decimal currency system, as well as shut down the London minting facilities and transfer to Llantrisant, a small village about 10 kilometres from Cardiff, Wales.

A lot of the Mint employees were very unhappy about having their jobs being transferred outside the conveniences of London, and some protested because of this.

It was during this period that the production of mint errors greatly increased. Quite a large proportion of the error production was deliberate.

Some of these error coins were even distributed (illegally) to the public for free! They were deliberately produced, and thrown by employees over the perimeter walls of the London Mint premises, to be picked up by passers by. Other error coins were simply allowed to slip through checking process, into general circulation.

I have a perfectly struck 1965 British Penny of about 2/3rds of the correct weight, which is common enough.
With a date of 1965, most probably a deliberate error.