I see from the information provided by Don that most rotary press stamps measure 22.5mm which it appears the margins measure on this particular stamp. Are there any other ways of identifying a rotary versus flat plate for this particular issue?
Measuring is notoriously unreliable. Flat plate will often have offset ink on the back. The best way is to compare to a known flat plate stamp. Line up the edges. A rotary design will be noticeable stretched.