It was a lightweight 37mm cannon taken from a derelict P-39 Airacobra. A lot of the PT’s were retrofitting cannons to be able to engage armed Japanese barges. The barges had too shallow a draft to be attacked by torpedo. So the PT’s were slowly becoming gunboats. Similar thing happened with the PT Boats operating in the Mediterranean Theater.
You are probably right but it sure seems he said 75mm.