Howdy
Definitely a Black Powder Iver Johnson. Sometime around the turn of the Century (1900) Iver Johnson completely redesigned their line of revolvers. In addition to using better steel, to withstand with the pressures of Smokeless ammunition, there were several design changes.
Three ways to tell a Black Powder Iver Johnson from a Smokeless one.
On a Smokeless Iver Johnson the little owl faces backwards on the grips, the hammer spring is a coil spring, and the cylinder locking slots have a hard edge both top and bottom.
This is a Smokeless Iver Johnson. Yes, the grips could be from another gun, but this one has a coil main spring.
The easiest way to tell is the configuration of the cylinder locking slots. Notice the difference in the appearance of these locking slots to the OP's gun. With the BP guns, the bolt only locked the cylinder in one direction, the hand kept the cylinder from rolling backwards. With this configuration, the bolt was captured on both sides by the locking slots.
According to the BATF, any firearm manufactured before 1899 can be considered an antique. Period. No, I do not have the date of that revolver handy, but that is the definition of an antique firearm in the US.
It is correct that prior to the Gun Control Act of 1968, Serial Numbers were not required on firearms. However most of the quality manufacturers, Colt, Smith & Wesson, Winchester, Marlin, and most others were putting Serial Numbers on their firearms from the beginning. I do have an old Stevens Side X Side shotgun that does not have a SN, so clearly it was made before 1968. However it is not an antique.