Looks like the enacting sections of Pub. L. 89-387 might act as a grandfather
clause. Couldn’t find the full text of that law online, but I found this:
This Act (enacting this section and sections 260a, 266, and 267 of this title and amending sections 261 to 263 of this title) shall take effect on April 1, 1967; except that if any State . . . or any political subdivision thereof, observes daylight saving time in the year 1966, such time shall advance the standard time otherwise applicable in such place by one hour[.]
So, if the eastern counties were observing Daylight Saving time in 1966, they might be grandfathered in even if the other eastern time zone Indiana counties didn’t observe DST.