"It's a plaque for the head tube of a bicycle frame from the Ajax cycle brand. That's why it's slightly convex.

The cycles with this logo were made in the town of Albert in the Somme region between 1906-1913, just before WW I broke out; 2 years later both factory and town were practically levelled by German artillery fire, and nearly a million men were k*lled at the Battle of the Somme there a year after"