I have a set of front aperture inserts from Champions Choice but need one larger than the 4.0 for the pistol target.
Gehmann has the 0.0x diopter separately, but I'm not finding a price for it anywhere.https://gehmann.com/english/products.php?id=1639
I would call the lens the front part of the assembly instead of the back though! In the sense that it goes closer to the muzzle.