Those were referred to as "rail mount" scope and bases. They were an attempt to have a secure system to mount the scope and the cross screws prevented it from moving under recoil. The also spawned the term "recoil proof" scopes . Like the other posters say TC has moved on from this system to regular base system for weaver type mounts.They were perfectly good scopes , just not popular. They will fit the hole pattern on an Encore but , because the base is not the correct radius , not stay attached very long.