Hi Mauro,
That's an interesting question. As you observe, the two units are very similar. The main difference is that the AC + DC Enhanced Power Injector for PTP 650/700 has a higher output current rating (1.7 A) than the PIDU Plus (1.4 A). The higher output current is necessary to support the power-over-Ethernet output at the Aux Port of the PTP 650/700.
The reason you should not use the PIDU Plus with the PTP 650/700 is (a) it doesn't have sufficient output current capacity to support the auxilliary PoE output, and (b) EMC and safety approvals for PTP 650/700 are granted for a system using the AC + DC Enhanced Power Injector.
For anybody tempted to use the AC + DC Enahnced Power Injector with PTP 500/600, note that it does not have a Recovery button.
Mark