Hi Rob and Dan, Terry from MRT. The simple answer is the 'bare' mPTX has a square circuit board and the older PTX has a rectangular one with cut-out corners. The old PTX 'old system only' transponders don't flash the LED when first powered up, the newer NS old/new system compatible version PTX (NS) displays two flashes on the LED when powered up.

The LED is neon blue on mPTX transponders,on standard spec PTX transponders it's blue but on custom spec PTX transponders the LED colour can be red, green, amber, yellow, purple, turquoise, pink or white, in fact the full spectrum was available. Nowadays we no longer offer customised LED's, the only option is the mPTX Pro with a 'pro' stealth all black wire power lead.

Terry_S