Caution:
Scott 423C (formerly 428a) is very rare and can be faked by re-perforating the top
and bottom of the vertical coil Scott 447 or, more likely, a margin
copy of 428. This stamp should NEVER be purchased, or
sold for that matter, without certification.
The September
13, 2000 Robert A. Siegel Auction Galleries Sale 826 has a
nice collection of the perf 12 x 10 errors.