D. OSBORN;140496
in one of the labcoat promo pics, venkman has the PKE in a front coat pocket. i have a pic somewhere of venkman holding the PKE in the courtroom scene of GB2…

Nix;140385
It's too small to be a paper-feed; in at least one of the storybooks you can see a still taken from the “advertisement” where it's clearly clipped to his lab coat lapels. A financial calculator has many buttons of indeterminate purpose, so this could be a case of “if it looks vaguely technical, we get to use it.” Same could be said of the Ghost Sniffer.

PS: I just realized that the calculator could be an early scientific calculator, if that helps.