You are correct with Birmingham, and the year appears to be 1925. Where on the box was the mark you show as the makers mark. There is an illegible mark on the left hand side of the anchor which is where the makers mark usually occurs - can you tell what that mark is?
I can only make out that mark when I look at it with a 10* loupe with an angle larkfield and the first line looks like C A G A few things with the shape of this mark (two straight lines with spikes on top) appear on the Birmingham page but none with the letters I'm seeing.