I was wondering this myself. I think that they were still tiling little metal blocks, according to this picture: http://www.datadeluge.com/2010/09/how-maths-was-typeset-befo...

Which makes it all the more impressive--each page took considerable craft and attention to detail.

For more information on this, you'd have to get a book like this one--I can't find anything more online: http://www.worldcat.org/title/printing-of-mathematics-aids-f...