Final answer:
The U.S. patent system does not have 'Strict eligibility criteria' as a unique feature; other features such as 'Disclosure of inventions', 'Limited duration of patent rights', and 'Encouragement of innovation' are fundamental to the system.
Step-by-step explanation:
The unique feature that was NOT a part of the U.S. patent system historically is d) Strict eligibility criteria. The other features like a) disclosure of inventions, b) limited duration of patent rights, and c) encouragement of innovation have been fundamental aspects of the U.S. patent system. The system was designed to promote innovation by providing inventors with the exclusive right to their inventions for a limited time, historically set at 20 years, to allow for a period of monopoly to recoup the investment in research and development (R&D). However, the system does not have extremely strict eligibility criteria that would be considered exceptional or unique; rather, it allows for a wide range of inventions to be patented as long as they meet the criteria of being new, non-obvious, and useful.