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so basicly the answer is to find ones that arnt 11 x 10 1/2 ?
Jim, You can use the perforations to determine which base stamp is behind the precancel, but it won't be much help in evaluating the value of the stamp. The vast majority of all printings (1922 perf 11, 1923 perf 10 and 1926 perf 11x10˝) will have very little vale. A few precancels will have more value because they are scarce, not due to the base stamp so much. In general a scarce precancel on a 1922 issue is also scarce on the 1923 issue...
You would be better off to ask about the precancels you have, so someone can look them up for you, or obtain the precancel catalog for yourself so you can look them up yourself. An older edition of the catalog would work just fine for you, since what you need to find out is which are scarce, and they are just as scarce now as they were 10 years ago...