Gas tank sending units and caps

["m williams" <gturismo62@domain.elided>]

Would someone please clarify some confusing questions for me?
1.  I was told by a mechanic who says he knows a lot about Alfas(?), that 
the 105 spider and GTV take different gas tank sending units.  Is this true, 
and why?
2.  He also proceeded to say that the specific sending unit will only work 
properly with a "bayonet" gas cap, instead of the screw-on type!!  Now that 
really sounds bizarre!!  Could this possibly be true, and how could the type 
of cap make any difference at all as to the way the sender detects the 
amount of petrol in the tank?  Please help clarify what appears to me to be 
an old wives' tale.  I am not knowledgeable enough about Alfas to disagree 
with a mechanic---yet!!  Thanks much.
Mark
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