Your button appears to have a version of self-shank back which I call a "pierced dome." See photo below showing a non-excavated example. For anybody here who doesn't already know... "self-shank back" is the term used by button collectors for a button whose back itself forms the shank (or loop) for the thread to pass through... instead of the back having a separately-made and applied shank/loop. Being far less expensive to manufacture, self-shank backs are found almost entirely on cheap-quality buttons.
The "pierced dome" version of self-shank back seems to have been patented and introduced sometime between 1900 and 1910.
All of that being said... the "pierced dome" self-shank back is seldom seen today, because the simple hole in a sheet-metal back tended to soon cut through the thread which held the button onto the clothing. Therefore, within a few decades the "pierced dome" back was superceded by an improved version of self-shank back, shown in the second photo. So, I think your button with a "pierced dome" back is more likely to be from the first half of the 20th Century than the second half.
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