In Topps' earliest annual report available online (1997), under "Production", they detail there that:
'High-quality paperboard is sent directly to outside printers by the Company's suppliers."
http://getfilings.com/o0000812076-97-000007.html
Annual reports are available for every year since Topps went public in 1987. If someone were to acquire a report for any of the years from 1987-1994, it should detail their production methods in the same manner in which it did in 1997.