It seems that Mark's excellent Sys 3-7 guides at pinball.flippers.info are not currently available. DNS still resolves, but requests seem to time out.
In the meantime, Mark's guides can be accessed via the wayback machine: https://web.archive.org/web/20221129000442/http://pinball.flippers.info/system6resources.asp
I exported PDFs of each page, and am mirroring the site w/ wget, just in case, as it would be a shame if all of this great information was lost.