Cassy is Charles Austen's daughter, not Jane's sister Cassandra. Jane reverses the letters within each word while keeping the words in their usual order: “Ym raed Yssac” reads “My dear Cassy.” The little Cassy whose third birthday is mentioned is another child, not the recipient. The Morgan catalogue dates the surviving letter to 8 January 1817, Le Faye 148. The unusual spelling is the game, not ordinary OCR damage.

Jane Austen to Cassandra Esten Austen

JAN 8, 1817

Ym raed Yssac,—I hsiw uoy a yppah wen raey. Ruoy xis snisuoc emac ereh yadretsey, dna dah hcae a eceip fo ekac. Siht si elttil Yssac's yadhtrib, dna ehs si eerht sraey dlo. Knarf sah nugeb gninrael Nital ew deef eht Nibor yreve gninrom. Yllas netfo seriuqne retfa uoy. Yllas Mahneb sah tog a wen neerg nwog. Teirrah Thgink semoc yreve yad ot daer ot Tnua Ardnassac. Doog eyb ym raed Yssac.

Tnua Ardnassac sdnes reh tseb evol, dna os ew od lla.

Ruoy etanoitceffa tnua, Enaj Netsua.

Notwahc, Naj. 8.

The words in this letter are deliberately written backwards. View the manuscript at the Morgan Library & Museum. Jane to Cassandra Esten Austen, January 8, 1817; MA 1034.6

Source: Woolsey selection, 1908 printing, letter 76.