This is a Truglo Sight known as the Fat-Bead, model TG948UD.
Large diameter" CNC-machined front sight that incorporates a dual-color light gathering fiber optic rod for high visibility and faster sight acquisition" even in poor lighting. Universal design fits most shotguns. Gunsmith installation may be required. Fiber diameter is .120" and length is 0.5". Includes mounting screws.
Features:
Their largest diameter bead replacement
CNC-machined
Fiber diameter is .120";Fiber length is 0.5;Dual color fiber optic red/green