That's interesting you should ask because I bought an E. Arizaga this year as well and have been looking for more info. Because the shop closed before the internet there is not a lot of information on the web. Eusibio died and the firm, "Successors of Eusibio Arizaga" was absorbed in the '80's into the collective DIARM that ultimately failed. Jack O'Connor sang his praises in
The Shotgun Book of the 28ga and 20ga sxs that he custom ordered in early sixties.
I have Terry Wieland's book,
Spanish Best, and its great perspective on Basque gunmaking but not a lot of specific info on Eusibio. Believed he worked in Placencia de las armas rather than Eibar.
You show us yours I'll show you mine....have since this photo changed buttpad. Gun is in the shop now getting a makeover.