Based on what I've read over the years here from various Colonial Era button experts, I think your brass solid-cast flatbutton with integrally-cast drilled shank is from the late-1600s into the early 1700s. By the mid-1700s, soldered wire loop shanks were becoming commonplace, as seen on "blowhole" buttons from that time period.
Also from wht I've seen posted here over the years, your cast-&-drilled 1-piece brass flatbutton is rare. There is some indication that type may be French-made, or Spanish-made. The fact that there were few French and Spanish colonists in America compared to the large number of British colonists may explain the rarity of your button. Seems like if it was British or American-made, we'd find a lot more of them than we do.
I forgot to mention, in researching your button I came across an old T-Net discussion of early-1700s "blowhole" buttons. A couple of drilled-shank brass solid-cast flatbuttons like yours show up among the various posters' photos of the blowhole buttons they've dug.
http://www.treasurenet.com/forums/my-best-finds/216095-blowhole-buttons-only.html