In reply to:

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I'll try removing the bell-wire connections at the master socket end and see how things go.

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Bell wire removed at master socket with two-wire microfilter connection between O2 box & extension socket :

Bandwidth (Up/Down) [kbps/kbps]: 1,308 / 21,273

Data Transferred (Sent/Received) [kB/kB]: 0.00 / 0.00

Output Power (Up/Down) [dBm]: 12.0 / 18.0

Line Attenuation (Up/Down) [dB]: 8.0 / 17.0

SN Margin (Up/Down) [dB]: 7.0 / 6.5
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I had to reset around 11am which may have given the higher sync, attenuation the same as previous post (two-wire microfilter connection between O2 box & extension socket, master socket fully connected). But very pleasing.