Though there are no known catalog references to factory-cased movements prior to 1900, several complete Model 1 and Model 3 watches have been reported in nickel Seth Thomas-marked cases with their distinctive square-diamond logo. Whether these were made in-house at the factory or by an existing casemaker is not known, but it seems unlikely that any case manufacturer would forego their own logo in favor of another brand. So far no early movements have been reported in gold or gold-filled cases of any kind.
The first mention of factory-issued cases is in the 1904 catalog with regard to the Model 8 Eagle Series movements being able to fit into the new "snug" cases. The Centennial Line, introduced in 1913 to mark the company's 100th anniversary, were all advertised as being factory cased.
This price list was found with a hand-written date of September 1, 1888, and given the existence of several very early complete watches that date is likely accurate. Note that the nickel and silver cases do not mention a brand or maker, meaning that they could've been made at the factory. Case manufacturing would've been a large parallel operation, requiring a substantial commitment before knowing if the pocket watch venture was successful or not. The gold or gold-filled cases were supplied by established case-makers; the Lion and Boss cases were made by the Philadelphia/Keystone Co, while the solid gold ones came from the Bay State Co in Massachusetts.
The 1913 Trade List featured the Centennial Line in all three of its sizes and clearly states that the watches were sold cased. They were offered in either plain nickel-silver or in four choices of patterned or smooth gold-filled cases with two levels of guarantee. They were advertised as being "the greatest value ever offered the trade and the public at these prices."
The final Seth Thomas catalog to feature pocket watches was the 1914 supplemental one, which at that point contained only the three sizes of Centennial watches. The case choices and their prices didn't change from those offered in the 1913 catalog.
The vast majority of 18-size Seth Thomas movements were uncased and sent individually in metal shipping tins to local jewelers to be cased in whatever style the customer chose.
These shipping tins offer valuable proof of serial number, jewel count, and grade - and what attributes defined them.