Thanks, but the forms weren't listed, and even a search on the IRS website didn't return a result. However, google brought up this:
https://www.taxaudit.com/irs-audit-letter-324c-sample-1
And what I got a month and a half later (a month after I sent the requested forms):
https://www.legacytaxresolutionservices.com/irs-tax-notices-letters/irs-letter-[product key removed]...
