It is described in the text accompanying the illustration as similar to S. cooperi. The other flower illustrated is labeled as Rhododendron campanulatum superbum, a hybrid recently introduced by the Kingston Nursery, in Surrey.
While S. cooperi is known to have lax flowering stems, it is unlikely that the live plant had flowers dangling upside down as shown here. I think it likely that the artist worked from preserved (pressed and dried) plant material and had not seen a live plant. The image probably reflects the appearance of the dried material.