The paper Rieger, L.
"On the consistency of the generalized continuum hypothesis."
Rozprawy Mat. 31 (1963) described in MR0146091, which can be downloaded from http://pldml.icm.edu.pl/pldml/element/bwmeta1.element.desklight-5ca4b532-4aa3-4856-924f-8bbed25e3c33 (click on the "31. książka") uses the phrase "potency set axiom" on pp 15, 20, 38 (and maybe elsewhere).
When discussing the "potency set axiom" Rieger refers to axiom C3 in Gödel's 1940
The Consistency of the Continuum Hypothesis
(Annals of Mathematics Studies, Princeton), whose text is reproduced in vol II of the 1990 edition of Gödel's Collected Works. Towards the end of Chapter II (on p.32 of the Collected Works) is a group of 4 axioms, labeled "Group C". Gödel describes the third of them as providing for the existence of a "set including the set of all subsets" of a given set, but does not use any of the terms "Potenzmenge", "power set" or "potency set".
So I suppose Rieger translated the usual term "Axiom der Potenzmenge" (used by Zermelo in his 1907 paper) into English, with the help of a dictionary, not knowing that "power set" is idiomatic and that "potency set" is never used. (See this for info about Rieger's life.)