I happened to google up this old (and resuscitated once already) thread because I recently saw a '53 180 listed for sale that still has the A engine. I remember once years ago meeting a older gentleman from BC who'd owned his early 180 for about 30 years. I asked which engine it had and he told me an A, when I asked why he'd never upgraded, he looked at me like I was crazy.
BTW from the Western Skyways website http://www.westernskyways.com/o-470/
"Remanufactured or Overhauled Continental Aircraft Engines: O-470-A, O-470-B, O-470-E, O-470-G, O-470-J, O-470-K, O-470-L, O-470-M, O-470-P, O-470-R, O-470-S, O-470-U"

