Native Americans counting in Brythonic

Deep in the letters and notes of 19th century folk of New England is a surprising fact: local Native American Indians counted one through twenty not in their native tongues but "een, teen, tother..." - which is ancient Brythonic Celtic counting used exclusively in Northern England and a close cousin of the Welsh language. As bizarre as this may seem, it's true.

From a paragraph of a letter dated 1876 and printed in 1879 from a certain Dr. Trumball to Alexander Ellis, then the vice-president of the Philological Society,

Some years ago, a writer for the Historical Magazine (the American Notes and Queries) called attention to a series of numerals formerly used by a tribe of Indians on the sea-coast of the State of Maine, and ‘handed down through the agency of a deeply graven tradition. (pg. 317)

That tribe Trumball mentioned was later identified as the extinct Wawenoc by one Joseph Lucas (1882:41), which was a branch of the Algonquin nation. But returning to the letter, Dr. Trumball said that local history records the Wawenoc using the counting method as early as 1717. Ellis incorrectly thought that the counting method stemmed from a mispronunciation of Welsh. (It cannot be, as de-aspiration of tan "two" from *dewi must have been ocurred at a very early date, indicating this was a different but related language from Welsh. To be fair, Ellis did admit that de-aspiration was a problem for his argument, but he was wrong nonetheless). Here's the counting method next to a traditional Lincolnshire count and its corresponding numerals in Welsh:

11-14.een/teen/...-gleegetyan/tan/...-a-dikun/tri/...-ar-ddeg except 12. deuddeg
16-19.een/teen/...-bumfrayan/tan/...-a-bumfitun/dau/...-ar-bymtheg except 18. deunaw

While the French had been in Maine since 1604, English settlement did not begin until the establishment of Portland in 1625, and even then it was small. It seems unusual that local Native Americans replaced their numbers with Northern English shepherd numbers within 82 years (as Trumball said the people had been using them since 1717). Furthermore, the count is limited to 20, which means it can't be a very useful method. If Trumball's account is accurate and truthful, then I think the Native Americans were using it as a novelty counting method, and not replacing their original numbers.

Ellis, Alexander J. "The Anglo-Cymric Score". Transactions of the Philological Society. Vol. 17, Is. 1. 1879.

Lucas, Joseph. Studies in Nidderdale. London: E. Stock. 1882.