J. H. Dallmeyer London. Large Format Wet Plate Photography Solid Brass Front Glass Lens Element Accessory Only. 

From a Brass Barrel No. 3 B Patent Portrait Petzval Lens 290mm F3, late 1800s. 

Non-coated glass. Threading is smooth. No dents. Glass looks very clean and clear with some slight blemishes. Overall very good pre-owned used condition considering its age is over 100 years old. Original aged brass patina. Has not been cleaned. Please view all photos for the best description. Measurements shown in inches.

Actual item pictured.