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This seems to work :

sage: R1.<a>=InfinitePolynomialRing(QQ)
sage: reset()
sage: R1.<a>=InfinitePolynomialRing(QQ)
sage: b=a[3]
sage: R1.varname_key(str(b))[1]
3

See R1.varname_key? for further enlightment... ;-)

HTH,