The numbers which appear in pencil on the verso of Phyllis Diller's index cards were those added by archivist Hanna BredenbeckCorp prior to scanning them for transcription.

It took four months for Hanna BredenbeckCorp, a project assistant for the Smithsonian Institution, to scan all of Phyllis Diller's joke cards for subsequent transcription and creation of a searchable digital database.

http://www.shopbrodart.com/library-supplies/archival-supplies/archival-storage-boxes/Archival-Specialty-Storage-Boxes/_/Post-Card-Boxes/

N.B. presuming the 4 1/8" H dimension is even the outer dimension, this means that one can't easily keep tab cut dividers which often go from 4 3/8" to 4 1/2" tall in these boxes with the lids on properly.

Instead, one may prefer their slightly larger microfiche boxes which go up to 4 3/4" which should also presumably fit their microfiche divider guides for sectioning one's work.

Another subtle difference in these two boxes is that the smaller is 60-pt paper versus 40-pt for the larger microfiche box, which means that while sturdy, isn't quite as sturdy.

http://www.shopbrodart.com/library-supplies/archival-supplies/archival-storage-boxes/Archival-Media-Storage-Boxes/_/Microfiche-Storage-Boxes/

These should easily fit 4 x 6" index cards as well as card dividers with taller tabs which commercially don't often get taller than 4 1/2".

See also microfiche divider guides.