Than Book has an interesting post tracing the Ashkenazic customs of the number of shophar blasts; from suggestions of 40, 60 or 100 in the 12th century to later suggestions like 60 (the Gra) to the actual practice of eastern and western European Ashkenazim in the early 19th century; 42 and 40 respectively. Than writes that "by the end of the 19th century, most poskim were pushing for 100 to become universal," and thus about a hundred years ago it began to be correct that 100 was "almost universal," as the Artscroll mahzor says it is today.
What motivated the search? A friend's dispute with a daughter's teacher over how many shophar blasts are traditional for Ashkenazim.
Here is an interesting example of how even what was the norm 100 years ago can easily be forgotten and supplanted by something new; then projected back onto the past.