For some sheets / panes of U.S. commemoratives which contain stamps of different designs Scott assigns a distinct catalog number for each stamp. For others Scott assigns a single catalog number for the set and a letter designation for each distinctive stamp.
I used to assume that the letter distinction was used when the different stamps of a pane together formed an image, such as with the pane of 10 "Nature of America: Hawaiian Rain Forest", Scott Number 4474 a - j issued 9/1/2010. But the pane of 12 "Modern Art in America", Scott Number 4748 a - l issued 3/7/2013 and the pane of 12 "Made in America: Building a Nation" Scott Number 4801 a – l follow this same convention.
Then there is the sheet (with two block of 8 sets) "Vintage Circus Posters" Scott Numbers 4898 – 4905 issued 5/5/2014 and the strip of 4 "Farmers Market" Scott Numbers 4912 – 4915 issued 8/7/2014 where each stamp is assigned its own Scott number.
There may be an obvious explanation but it is alluding me.
I think the previous post provides the now obvious explanation … if a pane consists of all different designs, the pane is assigned the major number & each individual stamp a minor number (with an appended letter suffix). Otherwise, the individual stamps get the major numbers while the smallest block containing all the designs is assigned a minor number. The single exception to this rule seems to be when the pane is a "booklet". E.g., 5125a is the 2016 Pets double-sided booklet pane while 5106 thru 5125 are the 20 individual stamps from the booklet pane.