At that time, the identity of the leeches was a mystery to me. Subsequently, one reader commented that they were either Hirudo verbana or Hirudo medicinalis. The latter is the "medicinal leech" that is still used to get rid of excess blood in certain medical conditions.
Today I came across a paper* from 2007 that gives the following picture to distinguish between the 2 species.
Fig. 1 from Siddall et al. 2007.
The patterns on the leech I photographed match those on Hirudo verbana.
According to the cited paper, molecular data demonstrate that leeches marketed as Hirudo medicinalis are actually Hirudo verbana. For details read the paper; it's available freely.
*Siddall et al. 2007. Diverse molecular data demonstrate that commercially available medicinal leeches are not Hirudo medicinalis. Proc. R. Soc. B 274:1481-1487. pdf