Tsk, tsk, Tom. One man's crap is another man's historical record.
Most of the correspondence will be 1950's versions of the newbie threads about circumnavigation/eclipses/seasons that we see every day.
I honestly don't expect much from this archive. This is a more knowledgeable Flat Earth Society than anything Shenton or Johnson ran. They weren't too technical.
Only Samuel Birley Rowbotham's original mid-1800's society exceeds us in technical merit. According to Flat Earth: The History of an Infamous Idea by Christine Garwood, Rowbohtam's society was publishing full scale scientific journals, holding regular meetings, conducting public experiments, and spent their spare time steamrolling RE astronomers in public debates.
The Universal Zetetic Society, the first Flat Earth Society, was composed of affluent doctors and professors who had the time and money to build the foundations of a Flat Earth. Rowbotham himself was very wealthy. He ran a medical clinic and a pharmaceutical company. He could afford to do the fundamental research and publish scientific journals - not just one, but several. In my research I've seen that over his lifetime Rowbotham was chief editor of at least 5 scientific journals dedicated to a Flat Earth: one dedicated to general flat earth subjects, one dedicated to astronomy, one to physics, one to earth science, and one dedicated to skepticism.
Earth Not a Globe is only a tiny fraction of his contributions to FET. It's a shame that those journals are lost and the public debates with leading scientists of the day went unrecorded.