Until you locate the sewer line, no one can do an as built.

The architect comment is related to my comment about engineers. Same restrictions apply.

I think it's clear that the lack of a written easement is not going to help you.

We don't know much about the sewer system in your area, whether it is public or private. It apparently dates to 1926 or earlier. At that age it's most likely a gravity sewer (it relies on gravity only). You might try checking with whoever operates it to see if they might have old plans showing the location of the taps. You can also check with them about the location of the first manhole downstream from you.

A dye test can narrow things down. Dye is flushed into the neighbor's line in their home. If the dye appears in effluent flowing at the deepest part of the next manhole (the flow line) then the tap is somewhere upstream from the manhole. If it appears in a lateral line entering the manhole from a pipe at an elevation above the bottom flow line, then you have found the tap as that line is the end of the neighbor's line.

In that situation, it is probable (but not certain) the the line is pretty much a straight line back to the exit point at the neighbor's house.

The test is not a do-it-yourself project. Ask the sewer utility.

Good Luck!