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.... and to dot the i's and cross the t's, by elimination S [4]'s alias
must be Donna d'Arc.

Very likely, but in that case, who is Sonja Ramada in Doppelstecker?

The same thought occurred to me but I thought the presence in both
films of the DD alias outweighed the possible coincidence of the
first-names.
As you imply though, it's a moot point until someone caps Doppelstecker
[unfortunately the purchase-link for Doppelstecker is in fact for
Doppel-Stecker].

Thanks.