I have a 5 gal Pin Lock Firestone corny keg that I don't have a setup for. The pictures are below...
The one above shows the indent where I guess the last bit of liquid would flow to the tube and the below shows how the bottom of the keg adjusts for the indent. After a lot of looking at it, I believe it is the nature of the keg and not a after production dent.
I haven't cleaned it since I never had a use for it. I would clean it for you if you did want it. I don't know anyone who has a pin lock setup to pressure test it.
I am looking for a ball lock keg to go with the current setup I have.