On a no-bodygrooves bullet, that flat base with a snipped-off circular mold-sprue is typical of
.54-caliber "Country Rifle" bullets. To see sideview and baseview photos of one, click on the following link and scroll most of the way down the webpage to the 9th-from-last bullet.
19th Century Bullet Collection - Tom Henrique
Or perhaps this link, going directly to that bullet, will work:
19th Century Bullet Collection - Tom Henrique
But you say your bullet's diameter is .570-inch. If you are sure that measurement is accurate, it can't be a .54 Country Rifle bullet... and no .57 or .58 Country Rifle bullet is listed in any of my bullet reference books. However, because there are several known calibers of Country/Picket rifle bullets (such as .40 and .42 and .54-caliber), I suppose there is at least a theoretical possibilty that somebody made a mold for .57 or .58-caliber "Country Rifle type" bullets.