So far as I can tell from various sources, the "Euskaro" marking is some sort of general appellation that was applied to various types of revolvers made by numerous different makers. I'm not sure which of those this particular trademark denotes, but I wouldn't be surprised if the "H" component stands for "hermanos" (Spanish for "brothers") as part of the company name, and there were several of these in the early 20th century ranging from the well known (such as Orbea Hermanos) to some very obscure and short-lived firms.

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SWCA #590
"Colligo ergo sum"