The EM172-Z1 and EM173 are physically identical, they share the same housing and diaphragm.
The EM172 has a 2 wire connection and is the easiest to use, it can be wired directly directly to most domestic recorders which have 3.5 mm microphone sockets and supply plug in power.
The EM173 has a 3 wire connection, its the 3 wire version of the EM172. It has no internal resistor from the source to ground, if you add a 100 ohm resistor between source and ground on the EM173 its identical to the EM172, of course you can select a higher value for less sensitivity.