US side documents are definitely the way to go to find out his origin town - Looks like his Naturalization is on Fold3. I've attached it here.
Bad news is the form at that time in PA doesn't appear to have any town info.
You said he was in PA for 11 years - if you let us know other locations and family names perhaps someone can locate or point out other possible resources.
Using the surround an conquer theory, it can be helpful to find other relations' documents to get this sort of info.
There are 1926 Pittsburgh naturalization documents for a "Michael Posta" in the Fold3 index. The original documents appear to have surname "Pesta" though, (but I wouldn't get hung up on variations.)
His birth town was "Zaluzice, Czecho Slovakia" and his last foreign residence was "Michaelovce, Czecho Slovakia."
I found 2 WWII "Old Man's" Draft cards for Anton Posta in Cleveland - at least the first does not appear to be your GF's as there is a naturalization for this fellow in 1922 on Ohio.
1. Born 21 Mar 1881 in Zbecno, Czechoslovakia.
Address 13615 Southview, Cleveland
Employer: Mr. W.H. Dick.
2. Born 06 Jun 1884 in Czechoslovakia (no town entered)
Address: 3878 E 151, Cleveland, OH
Employer: Strabley Baking Company.
But, including that latter info in the off chance they might be related, if not exactly his. Let me know and I can download these images as well. Good luck!