Looks like the unit used for this picture on Wikipedia https://en.wikipedia.org/wiki/HP-28_series#/media/File:HP_28.jpg https://en.wikipedia.org/wiki/HP-28_seri

Looks like the unit used for this picture on Wikipedia https://en.wikipedia.org/wiki/HP-28_series#/media/File:HP_28.jpg https://en.wikipedia.org/wiki/HP-28_series#/media/File:HP_28... could use one of these too (I assume the battery compartment door is in the upper right corner, where you can see some duct tape)