The Figures of some of Mr. Leeuwenhoecks Microscopical Observations, formerly publish't (in Numb. 94. p. 6037.6038) together with their Explication

Leeuwenhoek, A. van
Philosophical Transactions of the Royal Society
Royal Society

published 16 October 1673 (6 October O.S.) in Philosophical Transactions number 97, pages 6116-6118

At the beginning of the article excerpting Leeuwenhoek's description of the figures showing the structures he observed and discussed in his first letter, Oldenburg wrote:

This Curious observer [Leeuwenhoek], having been desired by the Publisher [Oldenburg], since his first Communication, already printed in these Papers, that, for further satisfaction, he would please to transmit the Figures of what he had so well observed, and he having not only very obligingly complied with that desire, but also added New Observations; we thought our selves bound to do him right in publishing both the Figures of his former Communications, and his Additions thereunto; though for this time we must content our selves only with the former of these, reserving the latter for another opportunity.

Even though the figures and explication seem to have come in a second letter, the manuscripts and figures are lost. The editors of Alle de Brieven / Collected Letters treated it as one letter. Leeuwenhoek's personal numbering system began with number 28, also treating these two articles and apparently two letters as one.