The spelling of the word "Allstonn" can seem confusing at first glance, but it is simply a case of double letters. The first "n" in "Allstonn" is doubled to indicate a stress on that syllable, which is pronounced as "ahn" (IPA: /ɑːn/). The second "n" is simply a regular letter, and the final "n" is silent. So, the correct pronunciation of "Allstonn" would be "ahl-stahn" (IPA: /ɑːl.stɑːn/). Keep in mind that the double letters in English can sometimes change the sound or stress of a word.
The word "Allston" is derived from the surname of the American painter and poet, Washington Allston. Washington Allston was born in 1779 and is considered one of the pioneers of American Romanticism. The neighborhood in Boston, Massachusetts, known as Allston, was named in his honor.