Here are some shots that highlight the variety of black subterminal patterning on primary 5 of adult Common Gulls. Primary 10 is the outermost primary and they are numbered in descending order in from the wing tip. Primary 1 is the innermost flight feather at the bend of the wing where the primaries meet the secondary feathers. Primary 5 is, therefore, the sixth flight feather in from the leading edge of the outer wing...
P5 on this wing has a complete black subterminal band.
P5 on this wing is reduced to a Z-shaped mark.
Here, P5 has an incomplete band, broken at the shaft of the feather.
The break is wider in this wing reducing the marks on P5 to two triangles.
The black marking on P5 can be reduced to a spot on the leading edge of the feather...
... or a black mark on the trailing edge.
Finally, there are plenty of Common Gulls that lack any black markings on P5, as in this case. Note that the black subterminal marking on P6 shows a hint of signs of a split here as well. Also note the complete white tip to P10, the merging white tip and mirror in P9 and the triangular white mirror in P8. What a great wingtip!