To determine the year of your Nissan outboard motor with the model number NS90A and the serial number 3B7-1 12374, you can refer to the manufacturer's guidelines or contact a Nissan Marine dealer for assistance. Typically, the first two digits of the serial number can indicate the year of manufacture. In this case, a serial number starting with "3B" suggests it was produced in the early 2000s, but for precise information, checking with an expert is recommended.