Pluto Explored Scott Number

There is a mini-sheet of four stamps, two each of Scott 5077 and 5078 plus a title on the top "Pluto-Explored". The Scott catalog shows a number of 5078a for a pair of these two stamps and just below that they list "Pane of 4, 2 each......" but they don't call it 5078b. Is the 4-stamp sheet also 5078a? I have noted similar situations a number of times on other stamp groups. Can you clear this up for me?

The sheet itself does not carry a unique Scott number. Scott only assigns the sheet a number if there is more than one design and the number of designs equals the number of stamps. The pair just has the "a" minor variety listing. Since the sheet has 4 stamps with 2 designs then each stamp has a different catalog number. It would be usually described as 5078-5078 Pluto sheet.

If the sheet has 4 different designs a 4 stamp sheet the assignment would be 5078 and each stamp would be 5078a,b,c, and d.

Thanks!