Does anyone have the similar " S W " hammered on their coin?

Hi, I have this United Kingdom, Half Penny from 1940.

On the obverse side of the coin there is the letters “ S W ” hammered or altered on the coin. Does anyone know why my coin has this lettering on?

Thank you

It's been counterstamped.

Unfortunately it's a common practice.

Maybe the initials of someone who was born that year?

Unless the person who did that sees this post, we may never know the reason.

Interesting, thank you very much. 

Sad to know that I will never find out. 

You're welcome.