Letter written on 7 Aug 1969 (to Kusum)
Osho wrote many letters to Ma Kusum Bharti and her husband Kapil, then of Ludhiana. Most were addressed to Kusum, but always mentioning Kapil too. All but a few were published in Prem Ke Swar (प्रेम के स्वर). The wiki has images of all but one of those published in PKS, plus a few more.
This letter is dated 7th August 1969 and was published in Prem Ke Swar (प्रेम के स्वर) as letter #4. The letterhead was a typical one of those days, showing a simple all-lower-case "acharya rajneesh" at "kamala nehru nagar" in "jabalapur (m. p.)", with his 4-digit phone number and a two-colour Jeevan Jagriti Kendra logo.
See also Letters to Kusum and Kapil