The answer is that it's an indicator that a piece is made of silver plate. Other similar terms are:
- EPNS
 - EP
 - EP over copper
 - EPBM
 - EPCA
 - EPGS
 - EPWM
 - EPNS A1
 - EPNS England
 - EPNS Sheffield
 - EPNS X1
 
See this article for a bit more information: What does 'EPNS' mean on my silver pieces?
  