I recall watching a documentary on these swords on, I think it was the Smithsonian channel...
Turns out, the ones inlaid with +ULFBERH+T are authentic, and the ones inlaid +ULFBERHT+ are period forgeries, likely trying to capitalize on the then highly regarded +ULFBERH+T swords. Still great swords, just not as good of steel as the genuine ones.
It was determined there too, that the steel ingots came form the middle eastern region, while the sword itself was constructed in the Nordic countries, as was expected.
**EDIT**
It was a special episode of NOVA, not Smithsonian.