I have what appears to be a XM-645 SPIW variation.
Headstamp is “I V I 6 9”. Instead of the standard green puller sabot it has a shorter WHITE sabot with a purple spot in the middle. The O/L is 2.387 compared to 2.483 for my green sabot specimen. The sabot sticks out of the case aprox. 0.139 (compared to 0.242 for the green sabot.). It is much flatter with an almost cylindical ogive.