Your find is what button-collectors call a "Nautical Motif" Fashion button. Fashion buttons are not Military buttons. They are made for use on "fashionable" clothing -- which the manufacturers want to look expensive so they put fancy buttons on the coats & jackets instead of simple functional buttons.
The majority (but not all) Fashion buttons come with an iron/steel back, because it's much cheaper than brass or aluminum. Typically, the iron/steel back rusts out after the button has been in the ground for a couple of decades.
For future reference:
No actual Navy uniform buttons have an iron/steel back. Frequent laundering plus salty maritime humid air will soon cause the iron/steel back to make a rust stain on the uniform.