"inter-library loan" vs. "interlibrary loan" Google Books Ngram Viewer data comparing across the full corpus (1870–2019), and it gives a real number: around 1925.
Here's what the data shows:
1870s...
2/2
The crossover: 1925. In the Ngram data, "interlibrary loan" (closed) first overtakes "inter-library loan" (hyphenated) in frequency around 1925, and from that point forward it never looks back — closed spelling stays dominant continuously through the rest of the 20th century.
(summary by claude)
The crossover: 1925. In the Ngram data, "interlibrary loan" (closed) first overtakes "inter-library loan" (hyphenated) in frequency around 1925, and from that point forward it never looks back — closed spelling stays dominant continuously through the rest of the 20th century.
(summary by claude)
1