Grumman Model G-5-175
G is the batch code. 5 indicates 50 mil hull thickness. 175 is the length. Batch G is 1960s, built in Marathon, NY before hull identification numbers were required. Without a HIN plate, there’s no way to determine the specific year.
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That is the Model number. Look for another tag or stamp near the stern. Grumman has stamped all boats since 1955 with a serial number, which was later modified to comply with the Coast Guard HIN when it was established. Marathon can tell you more if you can find the serial number.