Duggap is right. When a "coat size" (.8-inch or larger) navy type button has an iron back, it is almost certainly an imitation navy button manufactured for the "Fashion" civilian-clothing industry. (Button-collectors call that type a "Fashion" button.)
Extremey similar "Fashion" imitation navy buttons are still being made and sold today, at stores like JoAnn Fabrics & Crafts. Here are photos of some still on a store's sales-card. Note that the back of the card says "La Mode - the Fashion Choice."