Plain red Japanese Hinomaru. Nothing complicated here, but handy if you don't like painting circles freehand....
These decals are scaled to represent 30", 36" and 48" roundels in 1/300 scale. (They're also ideal for large aircraft in 1/600; equivalent to 60", 72" and 96" diameter.) The actual diameters are 2.5mm, 3mm and 4mm.