2015 Omega Seamaster Aqua Terra Day Date

Hello,

I'm new to this forum. I have a question regarding my Seamaster aqua terra ref 231.13.42.22.02.001
this watch has no serial number engraved on lug like most omegas do. Started to look around and found a few for sale with same variant as mine, does anyone know why this model only has serial number engraved inside of movement and not on back of lug?

Any information is appreciated.

thank you~