Updated Feb 8 - see further clarification from Benjamin Clark
In our interview with Gary Groth he mentions that when the Complete Peanuts was being made there was a strip that at the time had only a fragment left. Gary got permission from Mrs. Schulz to have Seth finish the strip. Then the complete strip was later found and subsequent editions of the volume were corrected. If you have a first edition of Volume 2 (53/54) you can see the strip in question on page 53.
I followed up with Benjamin Clark, curator of the Schulz Museum and he added this:
There was more than one! They had Seth patch up some missing bits ... I have the following dates:
Three of these four incomplete Sundays from The Complete Peanuts have since been found in contemporary clippings/microfilm in full. Only 10/2/1955 has not. Seth did nice, subtle title panels for these, using the title graphic for the era and adding a Schulz-ian line of grass. That's it. The added panels were very sympathetic, even subtle, that once you really look at you realize is definitely Seth. I only have a b/w microfilm scan of 10/2/1955, in the format that it appears in pretty much everywhere.
From Benjamin Clark Feb 8, 2024
There are several scanned black-and-white newspaper clippings. They are all in the format I attached earlier with the two tiers. The Museum only has black-and-white scans, nearly all identical. I sent you the best example. Sadly, we do not have any color scans or physical newspaper clippings of 10/2/1955.
The version printed in Complete Peanuts, with the Seth title panel -- Seth only did the title panel, not the whole top tier. So, where did they get the "missing" first tier of panels? I think they found them in an earlier reprint. The original strip was reformatted to fit in a comic book, which was reprinted in an issue of Tip-Top Comics in January 1957. That version has the existing 16 panels, but not the original title panel. I think that line art from that is what the current version is based on.
Attached is a scan from Tip Top of the reprinted 10/2/1955 from January 1957.