Very first though was 1916, but it doesn't look like the top portion extends far enough up to be a 6.
I was considering that too, but because the 0 on the 1910 was so large, i have a hard time envisioning it fitting in space between the 1 and the rim
Is there a VDB on the bottom of the bust? It would help narrow it down. Don't know that it would really matter.