My 340 swinger came with X heads it was produced in the 11th month of 69, and I believe a big reason that the x-head is more desirable is just because of the reputation of the 340 when it came out. It was still strong through 71 but began to fall off after that and still use the Jhead some people associate the lower numbers to the J head my belief is they are equal with 202 valves
