I missed this before too, I have what I thought were all the module datasheets here on my computer. What I missed is that the high density digital modules have their own datasheet.
So when you are looking at 0773 SNAP Digital Input Modules - none of the high density digital input modules are listed or mentioned. Perhaps it would be clearer if this datasheet was named something else or gave mention to the high density version?
It is probably better to refer to datasheet 1569 when looking for modules - or go through the website.