After letting this lie dormant for some time I have just bought Spencer Tracy - A Biography by James Curtis. The book is a weighty tome of 1001 page published in 2011 by Arrow Books
WRT Up The River Curtis writes that Tracy was signed to the Leo Morrison Agency on a six week contract commencing June 16th 1930 for $600 a week with a six months option on his services and the stipulation that he would be back in New York by August 21st as the show must open in Chicago September 1st.
Later:- 'Tracy was set to leave the Last Mile on May 26 when actor Lawrence Leslie, his replacement, fell ill with grippe. ... he was able to step away on Saturday June 7th, and left immediately for the West Coast. Despite the rush to get him to Los Angeles wher he arrived late Monday evening, Tracy found there was absolutely nothing to do'