I met a Beverley Sister

Bill asks why I think the 'a' in my cubic polynomial example of the previous post is a proper name.  Perhaps by analogy with the following dialogue.

A: I met one of the Beverley Sisters last night.
B: Really! Which one?
A: I forget. Let's call her 'Bev'.
B: OK.
A: Bev said she was born in 1927.
B: Ah. Bev must have been Teddie or Babs.

The logical constant 'a' seems to play the same role as 'Bev' in the dialogue as argument to predicate functions/subject of predicates.  And though we never find out exactly which Sister man A met, it's clear that 'Bev' is essentially singular.   A met only one woman that night.