What camera(s) produced negatives about 8mm x 18 mm in size. Three images per strip. They're likely pre-1940 era, not modern 110 film

Question

I've found what appear to be contact prints from small negatives. There are three images per strip, each being about 8mm x 18 mm in size. They were found in a processing envelope from 1941, so definitely not modern 110 format film. What kind(s) of camera(s) would have produced that size of negative?

Answer 1

Submintiature/16mm cameras like Minox Riga/A/B/C or Whittaker Micro 16 have negatives about that size.

The minox are often seen in movies and sold as "spy cameras" from that timeframe. The Riga model was introduced in 1938.