Probably the base of Shaft B as discussed at St M[arks] Book p.5.
This is the corner shaft of the southern portico nearest the campanile. Notebook M p.202L has a diagram (with ‘side note’) of the upper shafts above the capital on Shaft B. Sheet No. 129 has a plan of the disposition of the bases and pavement of the south west portico with the corner plinth noted as ‘shaft B’ i.e the column at the south west corner. See also Notebook M p.202 ‘Next, on capital of great shaft next Campanile; B p 180’.
[Version 0.05: May 2008]