In my books -7.3 is listed as given in J.C.McCoubrey,
Trans. Faraday Soc., 1955,
18, 1014
However, I recall seeing this year information that some group did experiments checking dissociation constants of strong acids and they found much smaller values.
OK, found it (sci.chem, posted by fkasner):
The advisor's name was Thomas Fraser Young (usually listed as T. F. Young) professor of chemistry a the U. of Chicago. The work was probably in the period of 1950 to 1960. The method of measuring the concentrations was Raman spectroscopy.
If you happen to locate these papers please post info here.