I called USSSA for two years ('95 and '96) as a favor to a local umpiring and volleyball mentor. Back then, there were VERY few differences between ASA's and USSSA's rules. The wording would be different, but the general effect of the rules ended up the same. USSSA tends to use more "common speak," while ASA tends to use more "legalese." 6 of one, half dozen the other.
Though times have certainly changed since then.
And yes, according to their website, they were chartered in 1968 in my home state of Wisconsin.