On P25 radios, Motorola has historically provided two control head generations of support. For example, the original Astro Spectra introduced us to the W-series control heads but users with high enough revision A-series control heads could actually use their existing heads with the radios. When the XTL5000 came out originally, the W-series heads were all that was available. Then Motorola introduced the Odyssey series mid-production run (about 2009 or so). So naturally, that was the control heads the APX shipped with as well and control heads like the MX on the XTL1500 were completely replaced by the O2 on the APX1500 and APX4500 with no backwards compatibility. Then the E5 came out and completely replaced the O5 with no official backwards compatibility (I hear it will work on an XTL5000 but have not confirmed that).