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There are very few rare pennies and especially within the dates that you have listed.
In 1918 some pennies were minted in alternative places and in the date aperture the letters H or KN can be seen on close inspection. They stand for Heaton and Kings Norton. When those pennies are in very good condition they sell for £5 to £10 each. Most pennies at that date are to worn though to collect.
The penny that you want (but cannot really have) is 1933. There were officially none minted but a very small number (probably less than 10) have been found to exist. A real one would be worth above £50000. They have been forged of course.
!951 was a low mintage but not very valuable.
So in summary, when large numbers of old pennies come to me don't even really bother to search through them.
Hope that helps a bit. Greg