Transmissions are not installed sequentially, so there is no precise VIN number that guarantees a non-snap-ring transmission. However, the snap-ring-range transmissions started getting installed beginning somewhere between VIN MT002500 and MT002600. Any '91 NSX with a VIN less than MT002500 was unlikely to have left the factory with a transmission in the snap ring range. However, if its transmission was replaced, it may have been replaced with one in the range. Again, the only way to be sure is to check the transmission number, but this will help you know the odds of finding one.
NSXs produced for other markets (Canada, Europe, etc) have different sequences of VIN numbers; the above only applies to those made for the U.S. market.