Register House

Register House is built to a design by Robert Adam, and according to the account in Modern Athens, the book that contained the above engraving, is "considered to be the most beautiful work of that eminent architect."  The building has a dome fifty feet in diameter and eighty feet high.

It is situated at the East End of Princes Street,

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facing to the south, towards the route into Edinburgh from the south across North Bridge.  It took nine years to build, and was completed in 1783.

Modern Athens quotes the following remarks that "appeared in one of the public prints":

"The public records of Scotland are kept in a dungeon, though a most magnificent building has been erected for their reception; but hitherto it has been unfinished and only occupied by pigeons.  Edinburgh may, indeed, boast of having the most magnificent pigeon house in Europe."

Today

Today, the building still holds public records, together with a private records that have been deposited, including early records of the Photogrpahic Society of Scotland.

One of the towers on Register House has dials similar to clock faces, but they are, in fact, dials for a weather vane showing the direction of the wind.  [This can be a windy corner of Edinburgh.]

Question

The other tower does have clock faces.  But on one of these faces, the numbers on the clock dial are:  1, 2, 3, 4, 5, 6, 7, 8. 9. 10, 12, 12.  Does anybody know why? 
[I believe that there is a reason for having two 12s, but I do not know it.]