As the reader, you can also ask mathoverflow for help finding the paper. There have been some very interesting MO questions along this line. In particular, this record helps others to also track down the same paper (or discover it never existed in the first place). Feel free to add to this list since this is CW:

• Scott-Solovay unpublished paper on ``Boolean valued models of set theory'' (never existed)
• Looking for a copy of Leo Harrington's unpublished notes on the first nonprojectible ordinal (now we have a version online)
• Harrington's unpublished note "The constructible reals can be anything" (now we have a version online)

As the reader, you can also ask mathoverflow for help finding the paper. There have been some very interesting MO questions along this line. In particular, this record helps others to also track down the same paper (or discover it never existed in the first place). Feel free to add to this list since this is CW:

• Scott-Solovay unpublished paper on ``Boolean valued models of set theory'' (never existed)
• Looking for a copy of Leo Harrington's unpublished notes on the first nonprojectible ordinal (now we have a version online)
• Harrington's unpublished note "The constructible reals can be anything" (now we have a version online)