If you accumulate a library of Strad and del Gesu pictues, or see the real thing, you learn that a lot of what you read in books about their spectacular discrimination in wood is -- ahem -- fiction.  They mostly used good to great looking quarter sawn wood, but you will find slab cuts (which can make fine instruments), knots, stains, no curl, irregular curl, and whatnot.  For example, the 1722 Jupiter Strad has a noticeable stain in the back.  The 1724 Avergemanny (sp?) Strad has pieces placed into the back where Strad evidently ran into defects in the wood and went to the trouble of getting rid of the bad spots,to avoid wasting a nice-looking back.  There is an early Strad with a huge KNOT in the back.   There are del Gesus with knots, as well.  When they could get and afford nice wood, they used it.   When it wasn't available or money was an issue, they used cheaper stuff.

Common names (eg maple, sycamore, hard maple, soft maple) are confusing and don't tell you the species.  Yes, Acer pseudoplatanus is found all over Europe and is the common violin sycamore/maple.  Medium-density American wood, Acer rubrum (Red Maple), can look identical to European maple/sycamore.  The weight and the working qaulities also appear pretty much the same.  What we in America often call "hard" maple (Sugar maple, Acer saccharum) is heavier and harder to work with.   No doubt plenty of violins have been with it, nonetheless.

BTW, your piece of wood looks quite nice, slab cut, but I don't concur that all makers would reject it.

Fred