In the counterlock assemby (A25) the reference signal is AC coupled (47 nF) and clamped to 0.6 V before driving the OSCIN input of a MC145146 PLL. Reference Output is driven by a NE5212 RF OpAmp, also AC coupled (10 nF). So I assume there will be enough power available to drive a second input, and there is definitely no 50 Ohm matching present.