S&P finally did respond to our query with a 100 page document. The part relevant to this question follow:
Select Sector Index Calculations
With the exception of the weighting constraints described above, each
Select Sector Index is calculated using the same methodology utilized
by S&P in calculating the S&P 500. In particular:
Each Select Sector Index is calculated using a base-weighted aggregate
methodology; that means the level of the Select Sector Index reflects
the total market value of all of its Component Stocks relative to a
particular base period. Statisticians refer to this type of index, one
with a set of combined variables (such as price and number of shares),
as a composite index.
The total market value of a company is determined by multiplying the
price of the stock by the number of common shares outstanding. An
indexed number is used to represent the results of the aggregate
market value calculation in order to make the value easier to work
with and track over time.
The daily calculation of each Select Sector Index is computed by
dividing the total market value of the companies in the Select Sector
Index by a number called the “Index Divisor.” By itself, the Index
Divisor is an arbitrary number. However, in the context of the
calculation of the Select Sector Index, it is the only link to the
original base period value of the Select Sector Index. The Index
Divisor keeps the Select Sector Index comparable over time and
adjustments to the Index Divisor ensure that there are no changes in
the Select Sector Index level as a result of non-market forces
(corporate actions, replacements of stocks in a Select Sector Index,
weighting changes, etc.).
Four times a year on a Friday close to the end of each calendar
quarter, the share totals of the companies in the S&P 500 are updated
by S&P. This information is utilized to update the share totals of
companies in each Select Sector Index. After the totals are updated,
the Index Divisor is adjusted to compensate for the net change in the
market value of the Select Sector Index.
Once a week the database containing the current common shares
outstanding for the S&P 500 companies is compared by S&P against the
shares outstanding used to actually calculate the S&P 500. Any
difference of 5% or more is screened for review by S&P. If
appropriate, a share change will be implemented by S&P after the close
of trading on the following Wednesday. Preannounced corporate actions
such as restructurings and recapitalizations can significantly change
a company's shares outstanding. Any changes over 5% are reviewed by
S&P and, when appropriate, an immediate adjustment is made to the
number of shares outstanding used to calculate the Select Sector
Index. Any adjustment made by S&P in shares outstanding will result in
a corresponding adjustment to each affected Select Sector Index.
S&P handles corporate actions which may arise from time to time and
which may have an impact on the calculation of the S&P 500 and,
consequently, on the calculation of the Select Sector Index. Corporate
actions such as a merger or acquisition, stock splits, spin- offs,
etc., require adjustments in the Select Sector Index calculation.
Index Divisor adjustments, calculated when necessary, are handled by
S&P in its maintenance of the S&P 500. In the event a merger or
acquisition changes the relative importance of a company's
participation in two or more sectors in a major way, the Select Sector
Index assignment of the stock may change. In any event, a new Index
Divisor for affected Select Sector Indexes will be disseminated
promptly by S&P.
If anyone wants the entire S&P document feel free to contact me by email.
Note this answer still does not provide a source for the data of when changes to the weightings (other than those by trading) occur.