That certainly appears to be a correct configuration beavertail for a post-1935 Ithaca. The earlier NID beavertails were short and fat but when they dropped the cocking indicators and went to the new style Field Grade they also changed the forearm fastening from just a Deeley to a Baker-Style J-Spring for the extractor guns and a combination of the Baaker-style J-Spring and a tilting block Deeley lever actuated forearm lock a-la-Parker Bros.
Here are some pics of original Ithaca beavertails from 1930 and 1936 Grade 4Es --
