Age of Yamaha with D before number?

[williamhawk]

What is the age of this U1 piano? Serial number D1744821. I'm confused by the letter before the numbers. On a British website it said the D meant the piano was manufactured in the 60's, and has two pedals, which this particular piano does have. But then it goes on to say that having a 7 digit serial number means it was manufactured in the 1970's. When typing in just the numbers, leaving out the letter, it says the piano was made in 1974.

Thank you to anyone that can help me out with my confusion! I would like to know when this piano was truly manufactured.

I didn't find the right solution from the Internet.
References http://forum.pianoworld.com/ubbthreads. ... ost2641582
Explainer Video Example

[Bill Kibby]

The majority of pianos cannot be reliably dated just by numbers, as I explain at
http://www.pianohistory.info/numbers.html
Near the bottom of that page, I list several Yamaha pianos that I have tuned, and verified the dates. You will have more chance of an accurate date if you look inside. See
http://www.pianohistory.info/datemarks.html

Various prefixes are used, and I don't know what N or D mean.