The icHeader.manufacturer tag is of type icSignature (a four-byte identifier value according to the ICC specification). The 4 bytes contain ASCII values; they are typically the first 4 characters of name of the printer manufacturer and, most importantly, are unique from other company names and correspond to the name that is used/reported by the printer driver. (For a more detailed discussion of how profiles and drivers are paired at run time, see Mapping Color Profiles to Printer Drivers. Examples values for this tag are "EPSO" for Epson, "CANO" for Canon, "HP " for Hewlett-Packard, and so on.
The deviceMfgDescTag value is also read and placed in the registry under the ManufacturerTag key for clarity even though it serves no functional purpose there.