Japanning (paint) was also used to stop the cheaply made thin iron backs from rusting although many buttons are found without the paint which was fired on but did not adhere well to the button back.
Many "victorian" type fancy buttons have this type of back.
Here's a closer view at the tombac (also shown above) back.
Notice the back retains the silvery grey color the metal originally had where the front shows brassing.
There is material from the shank insertion that sort of squishes out around the thick wire shank, this is typical of tombac shank construction.
This page shows the backs of various buttons which can help us identify them as civilian, work/overall and work usage by construction, materials and back marks. An RMDC is a backmark which consists of raised lettering set in a depressed channel. One way to get a handle on the age of a uniform button is to notice how the shank is attached.
Included are designer/vanity/blazer, uniform-look, work/overall and other look-alike types to help you identify the differences. Index to see some face designs of similar types (uniform, work/overall). This is a Scovill backmark, one of the most prolific button manufacturers in the U. These examples show how the shank is brazed at the base which is an older construction method compared to the inserted shank types where you cannot see any brazing compound.