The reason why I asked the source of your list is that Turtle Lake, Louisiana apparently never had a post office, but was only a place name, or a secondary name.
The oval device is not a government-issued type. Smaller offices had to obtain their own devices in this era, which this is. I am unable to narrow it down between Minnesota or Wisconsin, but I see paperhistory has solved that problem as we were simultaneously typing!
Add: here is the tracing from Billig's Handbook, vol X, page 178: