The Thunderbolt 4 stroke plugs were killed off years ago, and replaced by the BIG BORE 2 and 4 cycle plugs, both silver body both no idle bar. There is no big deal about idle bar plugs as you can get non idle bar plugs that work extremely well, The Merlin BiG Bore plugs work every bit as well as the Old T Bolt Big bore and are about 3 dollars cheaper, and they are extremely well made
I stock both of these plus the SIG RC long plugs
Randy