I've never seen Buttered Popcorn, but I've grown Spellbinder for over 20 years, and that looks like Spellbinder to me. In your climate there may not be much difference between a Sev. and a Dor., but even here in my Zone 9 garden, Spellbinder drops below ground before first frost (Thanksgiving) and stays that way until spring. That may help you distinguish between the two. The general form and poise of the flowers is another clue, (petal form, midrib and overlap) in addition to the color. Spellbinder is a brilliant gold, Buttered Popcorn is a yellow. Weather can affect all of these things though.
There's an "Epoch Factor" or "Era Factor" that has to be applied to daylily registrations. In 1957, green throats were rare, particularly in tetraploids, and so if there was even a hint of green deep in the throat of a flower on a cool spring morning, you might find "green throat" in the description. I wouldn't describe Spellbinder as having a green throat, and I presume that Sam Baker, were he alive today, probably wouldn't either.
"Nocturnal" is another term that has been misapplied in the past. At a time when early-opening daylilies were not that commonplace, and the term EMO hadn't yet been coined, hybridizers who wished to indicate that their flower began opening shortly after dusk dusk the day before blooming, and were wide-open before dawn often used the term "nocturnal-extended". The correct use of nocturnal indicates a flower which starts to open in the late afternoon and is generally spent by the dawn of the next day, or shortly afterward. Sometimes the morning sun has to actually hit the flower before it degrades noticeably.
"Self" is yet another tricky term. Some use it to indicate that the entire flower is one color, even to the depths of the throat, while others use it to indicate a flower with a single color on all segments, exclusive of the throat area, which is described separately, e.g. "red self with a green throat".
