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It appears Tellico got the year for you. If so,,, look on the top of the barrel to see if it has wording like; "Made in the 200th Year of American Liberty." If so, 1976 is correct.
As for value,, condition plays a big part, and if it's a true NRA graded 90% then it's strictly a shooter. Many folks would say a shooter like that would be worth the $300 you gave or a bit more since it's a convertable. If the condition is better than 90% the price can go up a bit.