icmn223Hey icmn,
It looks to me like the LTR series spark plugs have applications beyond just the US market, while the TR plugs are for domestic vehicles only.
The LTR5IX plugs have a thread length of 25mm while the TR6IX plugs have a thread length of 17.5mm. Also note that the LTR5IX plugs will run hotter than the one step colder TR6IX plugs.
I hope that helps!