Check out the pulp magazine archive website at
www.pulpmags.org.
They've put up a few early Black Mask magazines recently,
so that should give you an idea of what they looked like.
As a sample here is the August 1920 issue Vol 1 No 5
http://www.pulpmags.org/PDFs/BM_1920_08.pdf
You''ll find a few more at the same website.