The Howard F. Bowker Collection in the National Numismatic Museum, at the Smithsonian, has been digitized and is available on the NNM online collection – here’s the weblink (search for “Howard F Bowker”).
Here’s a screenshot of the first page:
For more information about Howard F. Bowker and his collection at the NNM, see Ellen Feingold’s blog-post “Exploring connections between the U.S. and East Asia through the Howard F. Bowker Numismatic Collection” (NMAH, 8 Nov 2017)
A search on the E-sylum for “Howard F Bowker” also produces many posts about this collector. Search here