Further, this page has some information about PET keyboards, but unfortunately not their circuit diagrams.
http://www.6502.org/users/andre/petindex/keyboards.html
In the section headed 'Keyboard Scan' it describes the key scanning operation as it would be in the diagram linked to in the previous post, and that does use a four to ten line decoder, ie, the 74LS145, to scan the columns. It's not clear whether the 'Business Keyboard' also employs a 4-to-10 line decoder to scan the columns as that is not explicitly mentioned in the following section, 'Business Keyboards'.