Volumes 18, 19, and 20 of Alle de Brieven / Collected Letters became available at the end of 2024, completing the series begun in 1939. Download for free.
Which number?
Master list (pdf) of all 602 L-numbers, their Leeuwenhoek number, and their Collected Letters number
Delft in 2010
2010
Screenshot from Google Earth, oriented with north at the top. Embedded map below.