Shunryu Suzuki's kanji for sesshin is on the cover of the certificate he gave to each participant in what he called The First Sesshin of Sokoji. It was two days in 1960 - February 20-21 - seven months after he'd arrived in the USA. A transcript of what he wrote has just been entered into the transcript archive with the filename 60-02-21. And here's a page for it on cuke - with a scan of the certificate. There's an overexposed Polaroid photo of the participants too.