The heads i have been seeing recently have JT followed by a single 3 digit number on the outside, and the same 3 digit number on the inside between the rocker studs....I am thinking you havent seen a new head in awhile maybe? If what your saying is correct, then what month/year are JT-05, JT-06 or JT-172 heads supposed to be made in? I have all 3 of those heads and all 3 of them have exactly what i wrote on the back, all 3 are legal AKRA clone heads that came on engines built by an engine builder and have passed tech at some point. This is why i dont agree with your post, The JT-05 and JT-06 heads didnt come out until a couple of years ago, 3 at the most, but by your post, it would mean they came out in 2005 and 2006, correct? Not trying to argue with you, im trying to make sense of where you are coming up with these numbers having anything to do with the date they were made