Summary:  Looked into the grounding issue as relates to the OMC and potential for coupling of ground related noise (including power line harmonics) to OMC length.

It is well known that there is a grounding anomaly inside HAM6 due to a metal cable bracket on the suspended OMC breadboard.  See:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=41709

A detailed analysis of the documentation (as listed on the attached PDF) indicates that there should not be such a coupling, however it appears as though there is.  This probably means there is an error in the documentation.  If this ground path exists, then a method is suggested where a single cut to the shield-to-backshell connection may result in a more favorable situation with respect to the OMC grounding and shielding.

A few quick measurements should be made (aided by the attached cartoon) to verify the actual grounding configuration, then a breakout board could be added to accomplish the needed shield break such that the theory could be put to test.  If there is a ground connection, as suspected, it would be interesting to know if the same situation exists at LLO, or if it's just a defect in the wiring exclusive to LHO.