mh-scan-msg-format-string

mh-scan-msg-format-string is a variable defined in `mh-scan.el'.
Its value is

"%d"

Documentation:
This is a format string for width of the message number in a scan format.

Use "0%d" for zero-filled message numbers. This variable is only
consulted if `mh-scan-format-file' is set to "Use MH-E scan
Format".