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If the DIARM came and went in the 1980's, can I assume Ugartechea's guns from the 1960's are good guality - no QC problems?
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All the Spanish shotgun makers, including Ignacio Ugartechea and DIARM, produced excellent quality guns. And all shotgun makers (Spanish, English, Belgian, German, etc.) occasionally have canine carnal knowledge of a gun they make. We’re no more likely to get a bad high end gun from one maker than from any other maker – regardless of nationality.
That said, there are several nice things to be said for buying a Spanish gun made in the 1960s. One of these nice things is any problems it might have had have long since been dealt with. Another nice thing is the 1960s is in that period of time when Spanish gun making had reached a peak of cost/value. A high end Ugartechea from the 1960s has a lot more skilled hand labor in it than the same gun made today.
As an aside, and entirely FWIW, the engraving on your gun is similar to that found on the Model 100 and model 119 during the 1940s – 1960s.