Kaktovik numerals (above) are a base-20 numeral system used by the Iñupiat people in northern Alaska. They were added to the Unicode standard last year.
Like a number of cultures around the world (e.g. ancient Maya and Dzongkha), the Iñupiat count in 20s. So, in Europe, did the ancient Celts, and their base-20 system survives in English in the phrase “four score and seven years ago” and in French in numbers like vingt = 20 and quatre-vingts = 80.
The Kaktovik numerals were invented in the 1990s by nine middle school students at Harold Kaveolook school in Kaktovik, Alaska. Like Maya numerals and Babylonian cuneiform numerals, they have internal structure, so that, for example, 17 is written as 3 × 5 + 2. This structure matches the spoken language, in that, for example, the number 17 is spoken as akimiaq malġuk (“fifteen-two”). Using extended European digits (e.g. 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, G, H, I, J) would not match the language in the same way – and, as noted by the students, could cause confusion.
Wikipedia claims benefits for arithmetic if Kaktovik numerals are used, but I have seen no studies on that (improved test scores for the school in 1997 may simply reflect the talent of this particular cohort of nine students). It seems like the system is no longer used in mathematics classrooms in Alaska, but is used for teaching numbers in the Iñupiaq language (where it no doubt emphasises the fact that mathematics existed in their culture before European settlement). Detailed records of the origin of the system were sadly destroyed in a fire at the school in 2020. However, if the numerals are made available on mobile phones, we may see a resurgence of interest in the scheme.