I also tried exporting one of my original test props as an OBJ, reimporting that, saving that as a new prop, and loading that.

Exported one of the Slicer swords as an OBJ, reimported it:

External Name = SlicerSword_3 (Internal Name = SlicerSword_3)

Saved as PP2

{
version
    {
    number 9
    }
prop SlicerSword_3
    {
    geomCustom 
        {
        ...
        } 
    }
prop SlicerSword_3
    {
    name    SlicerSword_3

New scene, deleted Andy, loaded 3 of theses

External Name = SlicerSword_1 (Internal Name = SlicerSword_3)
External Name = SlicerSword_2 (Internal Name = SlicerSword_4)
External Name = SlicerSword_3 (Internal Name = SlicerSword_5)

New scene, loaded 3 of these (not smartprops!), parented each to Andy,

External Name = SlicerSword_1 (Internal Name = SlicerSword_3:1)
External Name = SlicerSword_2 (Internal Name = SlicerSword_4:1)
External Name = SlicerSword_3 (Internal Name = SlicerSword_5:1)

The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

Forum: Poser - OFFICIAL

Subject: Why Does the External Name For A Prop SOMETIMES Have An '_1' Suffix