Without seeing this piston, it's tough to say what the cause was.
I highly doubt that any low speed fuel/air calibration would have any affect on this.
High Speed fuel metering....YES!
With our marine engines, and like Chief suggests, Ignition timing..... and in particular the progressive ignition advance, can and will cause Detonation.
Detonation may cause the piston damage that you describe.
Detonation and Pre-Ignition are not the same phenomenon. They are two entirely different issues.
Questions:
Did anyone check the progressive ignition advance before you set out to break-in this engine?
Or... did they set BASE advance ONLY, and turn the other cheek?
Which pistons are being used in this build?
If full dished, you have a battle right out of the gate, for any SBC Marine gasser.
Suggestion:
Run your ignition system on a Distributor Machine.
Watch and note the progressive advance.
Plot this out in graph form up to approximately 3.2k rpm.
Compare to your OEM specs for this particular engine build.
You may find your problem right there.
Examples:
Full In advance occurring too early as per rpm.
Too much TA (total advance) at the full in rpm.
Either may cause detonation.
What ever you do, check this out first, before you re-use this ignition system after the piston has been replaced.
You do not want a repeat as a result of an incorrect ignition system!
Perhaps post which ignition this is.
On the issue of the one piston only, this is rather odd, but does occassionally occur.
In order for only one cylinder to receive too much Ignition TA, or Ignition Advance too early, the cylinder firing seperation at the distributor would need to be off. That too would seem unlikely.
.