Very few ERAs were built, of any model. Most were built to comply with the pre-war 1500cc regulations, and most were supercharged sixes. Dick Seaman's famous "White Riley" is largely believed to be the "first" ERA. Interestingly, ERA's were still racing, and pretty competitively, in the immediate post-war years. The list of ERA pilots reads like a whos-who of prewar racing drivers.
-William
PS-if anyone has a copy, the British edition of Nick Mason's book "Into The Red" features a mighty nice sound clip of R10B.