Just a general thought:

Don't get too hung up on the idea that these 300 guns must have had some unusual or mysterious fate or destination because the impression rules that "very few" are currently known.

The number 300 is quite insignificant. It would be interesting to look at other comparable limited-quantity guns and the number of known survivors. Given that probably 99% of gun owners aren't into S&W history, most of the guns are likely out there with unsuspecting owners. What percentage, for example, of the 3000 Letterkenny-shipped US-stamped Colt DS's from 1970 are currently documented?

Remembering the list of Ft. Mason Victorys from Kevin's article, I don't think we need the shipment to have gone down with a ship as an explanation.