mystery 904 stall converter

Race converters are built for a specific application. Stall speeds can and will vary depending on what was done internally to the converter. Stall speeds on a 9" converter range from around 3700 to 4500, again depending on internal modifications. The number 11708-904 may be the serial number of the converter.
Unfortunately, unless the converter is cut open and inspected by a knowledgeable high stall converter builder, or you can contact the original builder of the converter it is really hard to say what stall a converter is.