Re: Year of 15 hp outboard
See that 'Y' in the bottom right hand corner? That is the year indicator for 'international' motors. Unfortunately, yamaha repeated the alphabet. 'Y' could be 1974 or 1995. The 'T' in the model number is the USA year reference, 1995.
I don't know (but really really want to know!) what the '2' means.
The first 3 digits of the serial number (684C) first appeared in 1981. It cannot be a 1974 model.
Your motor is a tiller, manual start, 15" leg, 1995 model.