Button ? Dug Up In Neighbour's Garden - Is It Silver ?

I think the Hallmarks are Britsh. The Anchor indicates Birmingham, The lion indicates Sterling and the e is 1903.....not 100% but pretty sure.

In England no item can be described as silver or gold without a hallmark... instead auctioneers must describe them as "white metal" or "yellow metal"

You say it is too light to be worth hallmarking, well how do you explain that every link on a watch chain from that peroid would be hallmarked individually? Factor into the equation that when you send a large batch of items for hallmarking the price really comes down (Last time I sent something for hallmarks was 1997 and it cost £7 for a single item but was pennies on a large batch of manufactured items)

The mark in the centre was where a jump ring was soldered... it could be half of a cufflink (two of these for each cuff with a short chain between them) I cant work out the scale it may be too big for a cufflink...in which case probably from a coat or blazer.

Neat find! Very ornate design. 1800's?

I would try a metal detecting dedicated message board. They have "help me identify" boards and you would be surprised how fast people show up to say exactly what an obscure item is, even with less markings than that!