I just did a quick check this evening for the 'Libery' net too - they are on 3947.5 KHz LSB tonight (2.5 KHz below the usual 3950 KHz spot) due to other traffic present on 3950 ...
The 'border blasters' started operating in the '30s, though not at the power that they did later.
I remember listening to one from Del Rio, TX when I was a kid in NE Illinois in the '40s.
Incidently, singer Slim Whitman got his start on the 'border blasters'.
I get a little magazine about old-time radio called 'Nostalgia Digest'.
The spring 2003 issue had an article about the 'border blasters'.
XERF from Ciudad Acuna, Mexico (over the river from Del Rio).