Re: Visas issued overseas (Riga, Latvia) in 1926 #latvia
From what you describe, your grandparents should have visa records on file with USCIS--the question may be where they are filed, or whether the search results you obtained were accurate.
I cannot check the manifest without the names, but assuming they were issued immigrant visas those documents would have been collected from them upon arrival in the US and forwarded for filing with the Immigration and Naturalization Service (INS, now USCIS) in Washington, DC. At that time the documents would become one of the Visa Files, 1924-1944, indexed at INS under the name as it appeared on the ship passenger list. If either of your grandparents never naturalized, or never interacted with the INS again after 1944, the visa documents should remain in their Visa Files. USCIS information on Visa Files is found here.
BUT, if they naturalized after March 31, 1944, their Visa File would have been moved to their Naturalization Certificate File, i.e., "C-File." The C-file, with the visa file contents inside, would then be indexed at INS under the name as it appeared on the naturalization certificate.
If not naturalized, but they had some other business with the INS after March 31, 1944, they should have an A-File (not simply the alien registration form document, but an actual file). The Visa File contents would have been placed inside the A-File, then indexed by INS under the name used at registration in the early 1940's.
INS' practice of moving files "up" to other file series (called "consolidation") can make requesting these files or understanding search results confusing. If one assumes that a post-1944 C-File only duplicates court naturalization records one can miss out on many vital documents.
All that said, there's always a chance your search results were incomplete. For example, if the only name provided was an Americanized name it may not have brought back a Visa File still indexed under the name as it appeared on the 1926 passenger list.
I hope this helps, you or someone else,