I believe all International Harvesters were made in 30.06, this is substantiated by the ".30".
If it was converted to .308/7.62 it would have a plastic block in the rear of the magazine to take up the space that would be freed up with the shorter rounds.
All USGI Garands that left the Springfield Armory, Winchester, Harrington & Richardson, and International Harvester plants as .30-06 rifles.
As far as the plastic block, I cannot confirm if that is true for Navy Trophy rifles. I know some gunsmiths do this on custom guns, but Navy Trophy rifles were military rebuilds.
BTW A_Navy: Check the bottom of the grip of the rifle. Does it have a letter stamped there?
I'm at work right now, but I have the past several issue of the GCA Journal in my office. I'm looking at an article in the Fall 2005 issue regarding the SecNav Trophy Rifle Call 7.62MM MK2 Mod 1 GR "A".
According to the article, the grade of the rifle's accuracy should be stamped on the bottom of the pistol grip area, "7.62 MM" was stamped/etched onto the barrel (in the area I described in my original reply), among other markings that may be unique to Navy trophy rifles.
Do you still have any of the accompanying documents with this rifle? Such as a hang tag, weapon record book, a DD 1149 form, or packing list.