In the Treatise, Maynard used E for Income; by Michaelmas 1933, he had 
shifted his usage to Y (see Rymes, Keynes's Lectures, 1932-35). As for 
where 
Hicks got I from, see Young, Interpreting Mr Keynes, where Hicks tells 
Meade 
on 6 Sept. 1936 (letter to Meade),that he is using Meade's notation (I for 
Nominal Income), Harrod's equations, and will pull his [CC-LL ]diagram out 
of a hat. In his diagram, reproduced from Arthur Brown's conference notes 
in 
Young, Hicks indeed has I on the horizontal axis, to be consistent with 
Meade's notation, as he said in his letter to Meade. 
 
Warren Young