I've looked into this many times. There is no hard fast rule on L78 rads and no clear usage pattern.
I've seen 4 core in L78's with 3.31 and 3.55 axles.
I've seen broadcast sheets call out ZD and the car got a 4-core.
I've seen 2 L78/3.73 built about the same month - earlier car has 4-core, later car has 3-core. Later 3.73/4.10 cars *seem* to get the 4-core, but not a lot of data.
btw, the PHR test car, 9L507834, had a 3.55 axle.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: William</div><div class="ubbcode-body">A few late May/early June COPOs were built with straight-neck 4-core radiators. Codes remain the same. </div></div>
I'm a doubter. The 68 4-core codes were the same and I've often seen 68 4-core service parts in cars. But I doubt they used 68 parts in mid-year 69.