A clever idea. Theoben’s proprietary mounting system doesn’t give many options for such well made and long-lasting guns, and short of buying an expensive new scope to fit my gun properly, these adapters allow me to fit more up-to-date mounts (NATO / 20mm).
So, maybe too early to give a long-term opinion as I’ve fitted them and the new mounts and used the gun only once since. Didn’t take long to get zeroed, and that might tell you all you need to know for now.