There are numerous of gaps in the catalog numbering. Many of them are due to renumbering. US Scott 691 is one of them.
691 used to be the 9 cent Thomas Jefferson, orange red shade of the 11x10.5 rotary printing, later combined with the listing for 641 now as "orange red, 1931".
The 691 numbering gap has been there since between the 1936 and 1939 catalog editions.
From the 1936 Scott US Specialized:
