Cbones,
I'm kind-a laughing here. You stated you checked the first four tires and the date codes are "1517" Now, normally, as you saw in the date codes of the last two tires "1312" & "1314", the numbers are laid out in such a manor that it's the week first and the year second. If true, that make your first four tires, made in the 15th week of 2017. Hmmmmm. So, your first statement of "1517" I'm assuming was maybe a bit of fat finger syndrome or, a basic Freudian slip. (I have those daily).
I'm presupposing that it should read " 1715 ".
Scott