I found a web site with date codes for Iroquois China. I'm not sure you are going to like it though - it indicates that the plate was made in Jan 1925 - or the first quarter of 1925.

Here is a link to the page:

http://www.restaurantwarecollectors.com/datecodes/iroquoisdates.html

They have date codes for lots of other china as well. I do not know how reliable they are....