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scott brockelmani would say that blank backs are a bit more common than the series of 150 backs, however i really never sat down and tracked them, but they seem to occur roughly 60/40 in favor of blank backs.
the overprinted backs are exceedingly rare, so much in fact one of the nearest to completion sets that exists only has 1 overprinted back. another of the larger holdings i have seen had about 15 overprints. in my set in progress i have 39 overprints an abnormally large amount. the few overprints that i have auctioned on ebay brought roughly double what a blank back or 150 back would bring, both were commons, a hofer with the overprint might go 3x's.
scott