An oval number plate as fitted to race bikes from the 1930s through until fairings became popular in the early 1960s. 
 
Made in aluminium, they are 280mm x 230mm and are left bare so you can paint it whichever colour is required. If sticking to UK ACU regs colours are as follows:
250cc - White numbers on a green background 
350cc - White numbers on a blue background
500cc - Black numbers on a yellow background