I can add info to what is above.
By: Wristenthusiast : December 2nd, 2019-07:41
If not mistaken: Also 1st series the leap year quadrants were 4,1,2,3. Later series became 1,2,3,4. Thus the leap year number coincides with the quarter of the year the months are in. I believe not all series have the day/ night shading on 24 hr indicator...