Strange extra f added to bytes object

[Robert Jackson]

I am very new to python so I'll apologize up front if this is some
boneheaded thing.  I am using python and pyserial to talk to an embedded
pic processor in a piece of scientific equipment.  I sometimes find the
when I construct the bytes object to write it adds an extra f to the first
byte.

For example if I have b'\x03\x66\x02\x01\xaa\xbb' it evaluates
to b'\x03f\x02\x01\xaa\xbb', which doesn't even seem valid.

Can anyone shine some light this?
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