25028A: M18 - 4.1mm ring diameter, 1.0mm ring thickness. £10
25028B: M22 - 4.8mm ring diameter, 1.0mm ring thickness. £10
25028C: M18 - 5.2mm ring diameter, 1.4mm ring thickness. Has some scratches but screws in OK when I checked it. £10
Premium glass elements for perfect sight picture
Screws into your existing tunnel, no clamp required
No refraction
Great for fullbore too
Polished flat glass with metal insert.