Plates for motorbikes were introduced in 1905 and they were very similar to the plates of the same age used on cars. The red number was the origin code (for instance, 55 was Rome... see table) and it was followed by a serial number in black. You can see a lead seal on the top left corner: I'm sorry if it is not very clear on this picture, but there's the print: "R.Prefettura N°55" (it means "Prefecture N°55"... the "R" probably stands for Royal): in fact, plate numbers were issued by prefectures. The big hole on the top right corner was probably made for the light or for a rear reflector of the motorbike.
You can find more about these very old plates in the page of plates for cars of the same time.
I need to thank Alessandro Libanore very much for this outstanding picture! You're great, Alex!