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?