For what it's worth, Apple are just conforming to the JEITA/CIPA DCF standard: https://en.wikipedia.org/wiki/Design_rule_for_Camera_File_sy...
“DCF file names” specification sez… http://www.kronometric.org/phot/std/DC-009-2010_E.pdf#page=2...
“File names conforming to the following rules are called DCF file names.
• The file name is 8 characters (not including the file extension).
• The first four characters consist only of the upper-case alphanumeric characters shown in Table 1
• These are referred to as the DCF file name Free characters. They shall not contain two-byte characters or special codes.
• The four characters that follow are a number between "0001" and "9999". "0000" shall not be used. These four digits are referred to as File number.
• Files with the same file number stored in the same DCF directory are considered to be object component files as defined in 4.3.2.”