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Made in Taiwan.
First 2 numbers in the serial indicates year of production (1990 or 1991). X in the serial indicates production Month (E to L - May to Dec). Y in the serial indicates production Month (A to D - Jan to April).
Bikes made in Taiwan usually had an 8 month production and shipping timeline before they were sold in bike shops around the world. In May of any year, factories would usually switch production to making models for the following year.