There is a freely-available "online reader interface" for this catalog on the HathiTrust web site at the URL below:

    https://babel.hathitrust.org/cgi/pt?id=uiug.30112019252946&view=1up&seq=1

Like the PDF accessible on the USDA portal page given in comments below, the HathiTrust page is somewhat slow to load...however, it offers a significant advantage. If one wishes to reference a specific page of the catalog, one can link directly to that page.

For instance, the URL below goes to pg. 1202:

    https://babel.hathitrust.org/cgi/pt?id=uiug.30112019252946&view=1up&seq=1206

If one wishes to link to "page n", simply replace the number following "seq=" (at the end of the URL above) with the value of n+4 (where n=1202 and n+4=1206 in the example above). Adding 4 adjusts for the pagination shift incurred by preface pages i–iv.

I had no idea about that correction paper. And searching for free, linkable online access to it...I found there were another two such papers! Below is bibliographic info for all three:

Sabrosky, C.W. (1967). Corrections to a “Catalog of the Diptera of America North of Mexico”. Bulletin of the Entomological Society of America, 13(2): 115-125 https://doi.org/10.1093/besa/13.2.115

Sabrosky, C.W. (1971). Additional corrections to a “Catalog of the Diptera of America north of Mexico”. Bulletin of the Entomological Society of America, 17(2): 83-88 https://doi.org/10.1093/besa/17.2.83

Sabrosky, C.W. (1978). A Third Set of Corrections to “A Catalog of the Diptera of America North of Mexico”. Bulletin of the Entomological Society of America, 24(2): 143-144. https://doi.org/10.1093/besa/24.2.143

Unfortunately, I couldn't find any freely accessible links for these correction papers.

Note there were corrections made in 1971 by C. W. Sabrosky: https://academic.oup.com/ae/article/17/2/83/211398.

This is online at USDA: http://naldc.nal.usda.gov/catalog/CAT87208336 Except for page 1202.