Fold3, formerly known a Footnote, is offering all of their World War II content free for the month of April. You can search from their World War II page.
I did a search on just the surname Feldstein. The first page of results were all draft registration cards from the fourth registration, known as the “Old Man’s Draft”, because it targeted men who were aged 45-64 at the time. The second page produced many draft registrations and a few other things, including one for Mathausen Death Books.
The draft registrations may help in the search for 1940 census records, providing addresses from April 1942 of men born between 28 April 1877 and 16 February 1897.