Linda,
 
Your keyhole impression on a small proportion of bricks is probably a tally
mark put on them during manufacture to assist in counting.  Brickmaking
teams were paid by piecework - ie, so much per brick - so they kept an
accurate count of how many they had made.  Every so many bricks - 20, 50,
100 - they  marked a brick to assist in counting.  It is not unusual to
find thumbprints on bricks, which at first look like the accidental result
of handling until they turn up in exactly the same place on a large number
of bricks, when they start to look like tally marks.  The impression you
describe sounds like a machine part, perhaps the end of a shaft or axle
with some sort of spline or key on one side.
 
Peter Bell
 
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