It's not a theatre knife but a WW2 utility knife made by
E.G.Waterman. The markings on the ricasso would be EGW/KNIFE, some were not marked.
This company made many thousands of these knives in 5,6 and 7 inch blade lengths and with wood or leather handles.
Some of these knives had a cap lifter cut into the bottom of the blade near the ricasso or had the relief cut like this knife.
All EGW knives look much the same and all were made during WW2.
They used to be the most inexpensive WW2 knife you could find with most going around $15-20 in EXC or better condition.
Hope this helps,
Warren
(orono)