What tool do I need to disassemble this 5 speed freewheel
I found a bunch of forum posts from people with similar problems. Shimano freewheels made prior to about 1985 have smaller diameters, about 20mm instead of 22-23mm that they have today.

A search for "Shimano freewheel remover 20mm" brings up some useful results. It looks like Bicycle Research Tools makes a remover called the CT-4 which is designed for older Shimano freeheels.

Good Luck!