Tough picture to scope, but this is my '78 D 100 custom... note the headlight/ turn signal arrangement. I THINK the '77 was the same. Rest I have seen the turn sigs were different. Can you get to a tail light and check the date on a lense?
77 and 78 were the same. My 78 I bought new had amber turn signal/marker lights. My buds 77 he bought used had the clear. I always liked the amber better, personally.
My 78 had amber grill lights. I actually bought amber aftermarket front lenses for my 93. I think the clear ones are too plain. The 78’s are my favorite grill, 79-80’s next, with the quad lights.