There's a story, propounded by someone who wrote it into Wikipedia 16 years ago, that the Norsk Data ND-505 was the world's only 28-bit computer, made so in order to avoid CoCom regulations then covering 32-bit minicomputers.

* https://en.wikipedia.org/wiki/Special:Diff/7310013

Unfortunately, the story doesn't agree with the contemporary marketing literature that still exists, that calls it a 29-bit computer; or with the CPU reference manual for the ND-500 CPU that was used in the ND-505, which says that it used 32-bit logical addressing with 5 domain selector bits and 27 address bits, and 41-bit physical addressing with 2KiB pages and a 30-bit page number.

* http://sintran.com/norsk-data/library/libpdpi/ND-505CX-A1-EN...

* https://archive.org/details/bitsavers_norskDataN00ReferenceM...

* https://archive.org/details/bitsavers_norskDataN00ReferenceM...