I installed it in a 400cc motorcycle because I do not want the back of the number to become dirty with chain oil etc. As it got thicker, I bought a 20 mm stainless steel screw + spring washer + washer at the home center and installed it.
Mounting on Narashino number. It is narrow and can not be installed as it is. I cut a few edges on the number side and managed somehow. Last time, when it was an adachi-numbered number it was installed without processing. The size of the number seems to be slightly different depending on the area.