An internet search on "Even stevens vs. even steven" it is noted that Even Stevens was an Australian horse that one two major cups in Melbourne in 1962. Even Stevens was a New Zealand horse owned by Sir James Wattie and brought to Melbourne for the cup carnival. He visited the company who employed me at that time. The phrase "Even Stevens" definitely gained prominence following his two significant wins. I have no recollection of ever hearing the phrase prior to 1962 although it may have been a dormant expression in Melbourne.