I found a carb website not long ago that had a lot of the codes for Ford carbs. I think it was a Mustang site (of course!) but might have some info pertinent to our stuff too. From that source, it seems that the C 5L was all about the size of the butterfiles, CFM rating, and jet size. Other than that, obviously the D6 means '76. T generally means Truck division, though they can be used on cars as well. And the E would be the revision number. Funny about the YA. I don't know what it means it "Ford-speak" but it's also a designation for a Carter carb type. I wonder if Carter made some Motorcraft carbs for Ford? Ya never know...
At least I think that's what all is. I'll go digging, but someone here might give you a better description.
I'll see if I can find that site and post a link.
Paul