There is a long history of portraying California as an island, and this may be one of the most obscure instances. The source is not known (though ca. mid/late 17th c), but it seems to me to be a depiction of Dutch East India Company medallions (depicting city views of Leyden, Harlem, and Breda), and the California map turns up on the reverse of one of them, as bottom.
The original is only about 1.5" in diameter--here it is, enlarged:
Antonio de la Ascensíon is credited with first giving flight to the idea that the California Peninsula was in fact an island--and so, beginning in 1620, California would frequently make guest appearances on maps as an island. (Fray Antonio was born in Salamanca in 1573/4 and studied there and at the College of Pilots in Seville; he was ordained in the Order of Discalced Carmelites, and sent off to Mexico. As a cosmographer he accompanied Sebastian Vizcaino (1548-1624) on his expedition to California to find a good port for Spanish galleons coming from Manila, which is when Antonio produced his diaries with the famous island maps.) This map was created during a period where there was heavy competition for trade routes and geographical knowledge was actual currency, so there was some amount of cartographic information that was proprietary. When the abbot's map was lost to/and recovered by the Dutch, the changes that were made to California were codified and published--the Spanish thought it not necessary to correct the misinformation. Perhaps. In any event, it seems plausible that the Spanish crown had no interest in involving itself with what may or may not have been the loss of proprietary information, and so let the island business slide. Until1751, when the King of Spain issued a proclamation saying that California was indeed not an island.
This practice continued to about 1747, when there was more than ample evidence to suggest that the island status of California was erroneous--still, it took several decades for the last of the island-maps to make its appearance.
- [California as an Island inset map, ca. 17th c.] Engraved image 10.5 x 5.5" on a 12x9" sheet; the California/Hemisphere section measures 2.5", only. Very good condition. $175