Bike Model

How do i tell the year/model of this dirt bike?

I know its a yamaha, and that its been in the woods at least 30-40 years. I do not know the model or the exact year. The registration sticker is gone. Alot of the bike is also missing. What is present are the frame, front wheel, front shocks, engine, oil tank, and exhaust. Is there anyway to tell what year or model it is?
I know its a yamaha, and that its been in the woods at least 30-40 years. I do not know the model or the exact year. The registration sticker is gone. Alot of the bike is also missing. What is present are the frame, front wheel, front shocks, engine, oil tank, and exhaust. The VIN number has also rusted off. Is there anyway to tell the year/model??

Look on the bottom of the frame.

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