This is an External Clock cable for GPSDO with SMA connector like LBE-1040 and MiniGPSDO  which allows the RSPdx, RSPduo, RSP2 or RSP2-R1 Master Oscillator to clocked from GPS Disciplined Oscillator instead from the internal RSP TCXO clock.   

The frequency stability of the LBE-1420 is  specified as +/- 1 part per 100 Billion (+/-1e-11) after lock is obtained versus +/- 0.5 parts per Million (0.5e-6) for the TCXO internal RSP clock, hence any frequency drift is reduced by a factor 50000

What does this mean in practice?  There are certain application when receiving transmissions in the L-Band  or higher UHF band where the RSP Receivers' receive frequency will shift by as high as 500 Hz or even more during the night in a cold room when the Central Heating is switched off.  When LBE-1040 output signal is used as external clock for the RSP Receivers, the Receive frequency will stable and drift less than 0.1 Hz, providing uninterrupted reception of wanted transmissions or frequency data collection for Astronomy or Scientific purposes over extended period of time in varying temperatures. 
This external cable is also suitable for the Leo Bodnar MiniGPSDO

User instructions         
The LBE-1420 External Clock Cable consists of a SMA Plug to MCX cable and a fitted 3dB attenuator SMA-SMA attenuator. 
The External clock input of the RSP radios is specified by SDRPlay as minimum 1V pkpk  to maximum of 3V pkp
The  RF output level of the LBE-1040 is approx +11.5 dBm (=3.3V pkpk over 50Ohm) but this level cannot be reduced unlike the MiniGPSDO or the 2 Port GPSDO.   
Therefore the supplied 3dB SMA SMA attenuator reduces the output level to about +8dBm (=2V pkpk)  which is right in accordance with SDRPlay specification. 
Reducing the RF output level does also reduce any  spurious signals generated as result of high levels of External Clock signal.

Note: For Leo Bodnar 2 Port GPSDO please order the BNC - MXC clock cable with 50R termination