http://www.r-type.org/exhib/abc0004.htm
That is mid period production. The early ones had a metal skirt on the base like Loctals and the last ones looked like B9A Novals but with a locating pip on the side. Mazda produced some with a central metal spigot on the metal skirt:
https://commons.wikimedia.org/wiki/F...da_Rimlock.JPG
They were largely restricted to the UK and Europe, but were the dominant standard there in the first half of the 50s.