Not 100% sure, but I suspect it has to do with the fact that Bullseye started out as the 'fines' from manufacture of another powder -- essentially, it was waste.
Grain size was irregular, as was the burning rate.
Hercules Bullseye was the first time the powder had been manufactured as a product in its own right, with disk size used to standardize its burning rate.
The 1903 Ideal handbook apparently has reloading information for the Laflin and Rand version of Bullseye.
Laflin and Rand also introduced Unique to the market in 1900.
Bullseye and Unique are the two oldest powders currently in production.
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