Within the ESA PLATO M3 mission, the Telescope Optical Unit (TOU), i.e. the opto-mechanical unit, is a fully refractive optical system. The 26 TOU Flight Models (FM) to be delivered to the upper level, the PLATO Camera, make it a series production. The first Flight Models production faced many initial challenges from a Product Assurance point of view, mostly related to MAIT activities, while moving forward these challenges decreased. Discrepancies and non-conformities associated with, mainly, but not only, materials and processes, cleanliness and contamination control, safety, qualifications and validations, are the object of this proceeding. Thus, showing that serial production adds one more variable to possible failures, but at the same time, when root causes are corrected and solved, yields less difficulties in subsequent FMs MAIT and final production. Product Assurance, in monitoring the product in failure-proofing aspects, aims at mitigating criticalities and arranging for corrective and preventive actions that allow improving the likelihood of success of the mission.