I was looking more closely at this early, quarterplate Lerebours et Secretan lens today, and noticed a glint of something at the edge of the rear glass. At first I thought it was a miniscule edge chip I'd not noticed before.

Then I looked closer....and closer... finally through a loupe. They had engraved the serial number on the glass face, not the usual French practice of pen/pencil marking on the ground edge itself. The number matches the barrel, a cool discovery about these earliest of makers (it's pre 1855). White thing in photo is a grain of rice for scale. Lerebours owners, better go check your glass.