This is a great picture and of course the picture that inspired the Pulitzer Prize for Photography. Very cool that your Dad was the photographer.

I have to say though that what he gave the goons were either empty or unexposed film packs, not glass plates. Other pictures of the incident show him shooting with a 4x5 Speed Graphic. He was either using two-shot film packs which carried two pieces of sheet film with removable dark slides. One shot. insert the dark slide flip the pack, remove the dark slide, second shot, insert the dark slide, reload. They also made multipacks that were a little faster to use, (although I don't know if they made them for the 4x5.)