There is a legal basis for the OP’s belief. A city’s “official” name is usually determined by reference to the legislation that establishes the city, or by the city charter or code. Those documents are fairly clear that the City’s “official” name is the “City of St. Louis,” and not “City of Saint Louis.” But many government departments use “Saint Louis” instead of “St. Louis.”
The Missouri Constitution provides that “the city of St. Louis, as now existing, is recognized both as a city and as a county unless otherwise changed in accordance with the provisions of this constitution” (art. VI, sec. 31). See http://www.moga.state.mo.us/const/A06031.HTM. The Constitution refers consisently to “St. Louis” and not “Saint Louis.”
The Missouri Revised Statutes likewise refer consistently to “St. Louis” and not “Saint Louis.” See http://www.moga.state.mo.us/homestat.asp. (The single exception is the Saint Louis Zoo, whose name the statutes ordinarily spell out, with one exception–section 301.3045–where the catchline abbreviates the name but the text spells it out. But maybe the zoo is named after the saint rather than the city?)
The City’s voters adopted its original municipal charter in 1914. The 1914 Charter provided in article I, “corporate name and powers,” that “the inhabitants of the City of St. Louis, as its limits now are or may hereafter be, shall be and continue a body corporate by name The City of St. Louis . . .” (art. I, sec. 1). See http://stlouis.missouri.org/government/Charter/partI.html#I. The 1914 Charter refers consisently to “St. Louis” and not “Saint Louis.” The current charter carries this provision forward, and even places the name in quotation marks: “The inhabitants of The City of St. Louis, as its limits now are or may hereafter be, shall be and continue a body corporate by name ‘The City of St. Louis,’ . . .” (art. I, sec. 1). See http://www.slpl.lib.mo.us/cco/charter/data/art01.htm.
But the municipal code is the “Revised Code of the City of Saint Louis, 1994, Annotated,” and it refers to “the Charter of the City of Saint Louis, 1914, as amended to April 18, 1994” (sec. 1.01.010). See http://www.slpl.lib.mo.us/cco/code/data/t0101.HTM. The code is subordinate to the charter, though, so the fact that it uses the name’s spelled-out form does not override the charter provision adopting the abbreviated form. And even the seal for which the Revised Code provides (ch. 1.16) bears the legend, “THE COMMON SEAL OF THE CITY OF ST. LOUIS.”