From previous experience (in UK Primary Schools doing Support late 1990's early 2000's) there were several types of connection in use, the majority used serial with an occasional proparety connection with an add-in card (I'm talking Win95/98 era). I even saw one connected via the 'Joystick port'.
There were 'overlay' type devices for CRT monitors which were very troublesome (mostly due to drivers/software), the best devices were integrated into CRT's and used a secondary serial cable to connect to a port. Some VGA cables were available integrating the serial connection - but were hugely expensive.
When I stopped supporting (end of contract) USB connected devices were becoming more of a standard - software and drivers were much better, although the monitors used were still CRT's with integrated touch overlays as TFT's were very new (and expensive). In theory any TFT could be 'converted' to touch by sending it away, but suppliers had preferred makes/models they tended to use.