I had several RD350's and RD400's.
That looks like a '76 to me.
I thought that orange was a '75 RD350 (I had one. VERY fast!), '76 400 was blue. '77 400 was Red (had 2 of them). '78 was gray or silverish, if I remember correctly. '79 was the Daytona Special (had one!!! shoulda kept that bad boy).
I am pretty sure I have the above correct. Unless Yamaha had multiple colors in given years.
edit: now I am remembering the tank graphics were larger in '76. This one has graphics like my 2 red 77's. So, maybe '78 had blue and silver.
It was a RD400D the D designated the year it was made. I'm pretty sure it was '78 but.................
First street legal bike anyway. Too fast and quirky for someones FIRST bike. Power band was from about 7000 to 7100rpm hehe.