According to Russo's
Collector's Guide, fifty years ago to-day (2 April 1971), Argent's "Celebration" b/w "Kingdom" (Ger. Epic EPC 7120) was released in the U.K. The U.S. and Canadian release (Epic 5-10746) was on 21 June 1971.
As far as I know, "Kingdom" hasn't been re-issued, and until looking through Russo's discography, I'd never heard of it, let alone heard it.