The date is the difference. On Japanese coins the date is based off the reign of the Emperor. The the title of the era is going to be different. Y45 is Year 9-15, the Taishō era (正大). Y49 is Year 2-7, the Showa era ('昭). The characters in the parenthesis follow the date on the coins and distinguish between the two eras.
But also remeber, the characters on Japanese coins of that era (and before) are read from right to left. So 正大 should be 大正 when written from left to right.
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