what i ment was a half moon conecter
i think i found one but it doesn't have instructions but has many pics go to this link https://www.instructables.com/id/my-new-knex-m4-carbine/
i dont know of any but there are many grenade launchers on this site. i have two myself. i am not really sure what you mean by circle yellows, but most of the lauchers use almost every kind of common piece.