What year 340 heads got hardened seats?

I ran my 340 X-head motor 10.000 + miles without hardened seats, I always checked my valve lash at least 2 times a year and never had to adjust them. That was a waste of time but If the exhaust closed up I would know it was burning a seat, The last time I had the motor apart the seats looked like new, In 20 years I had that motor apart many times to check it out and it never needed anything, many 7500 rpm 1/4 mile runs plus street driven. I love my mopars.