Attached are scans from Scott Lorimer's(Apollo) log book.  These graphically depict the plan view location of the elevation control available at the EndY and tabulates their value.  Nominally we would survey the BSC chamber (in this case BSC6) at the manufacturer's 90° marks on the B nozzles (BSC 60" door) (which we did) and then set the datum zero to the average of these eight elevations.  I elected to not do the latter as the average was less than 0.2mm approaching the noise of the measurements.  So the current end station elevation datum is zero at the South horizontal scribe of the East B nozzle.  Otherwise, use the elevation numbers from the table (in millimeters) to start your vertical survey.
Note, BM#101 is North of BSC10 and not seen on the plan view.