Is it a re-printing or a new edition?

The first editions of Leeuwenhoek's publications are fairly straightforward. After that, the situation gets murky. Apparently, Leeuwenhoek soon needed more copies than he had originally had printed. For reasons we don't know, he re-packaged the pamphlets in various combinations, sometimes with different printers. Visual comparisons show that the type was set again and that the figures were inset again.

One example:

Comparison of the first two pages from the 40th Missive

Ondervindingen en Beschouwingen
... vande Schobbens in de Mond
Antoni van Leeuwenhoeks 40ste Missive,
Geschreven aan de Heer Francois Aston
Gaesbeeck, 1684 unknown printer, unknown date, probably Boutesteyn

Dobell (1932, p. 392) dated the second one 1696: "[Lugd. Bat. 1696 ?] [No preliminary leaves : pp. 1-24, identical with 1st ed.]". Without any preliminary leaves, it's not clear why he thought it was printed in Leiden in 1696. It is not identical.

The earlier printing has a separate title page and a two-page preface from the printer to the reader. The later has no title page. At the top of page 1 (click to enlarge, move, and resize), the words "Antoni Leeuwenhoeks Onder-Vinding" are replaced with "Antoni van Leeuwenhoeks 40ste Missive." and the other two parts are reversed. It is the "van" that makes me consider it a later printing.

After that, the content is the same, but the typesetting isn't. The plates were inserted differently. The later added two punctuation marks in this excerpt alone: the hyphen at the end of the fourth-to-last line and the comma in the middle of the next-to-last line. Every page has similar differences.

In the addenda to his biography (1950, p. 509), Schierbeek gave his insight on Leeuwenhoek's working methods, also relating to the situation noted below about the difference between printings and editions.

Mr. P. W. v. d. Pas in The Hague informed me that he possesses a copy of an old Leeuwenhoek edition in which the letters 32, 33 and 39 are included. Letter 32 consists of two signatures of 4 pgs., Letter 33 of 8 such signatures, letter 39 of 5 signatures. Each letter is thus a whole number of signatures so that they could be bound in various ways. As all these signatures have additional holes that occur all at the same place, they at the time must have formed together a whole. Originally stitched together in that way. Mr. v. d. Pas therefore agrees with my view that the first issue consists of three letters 32, 33, and 39.

In short, Leeuwenhoek saw each letter as an individual unit that could be combined with other letters. That frame of mind lasted for about three years, from 1684 to 1687. After he got past the first two dozen letters (#28-52 in his numbering system), he began publishing them in increasingly larger bundles usually in chronological order with continuous pagination. By 1718, his last batch, 46 letters, all appeared in one volume, Send-Brieven. Like later children, these later letters did not get as much individual attention as Leeuwenhoek's first-born.