Interesting. The link on museumtour.com points to http://patft.uspto.gov/netacgi/nph-Parser?...841&RS=PN/5,933,841
which is US Patent #5,933,841, filed May 17, 1996 and issued August 3, 1999.

Cypherpunks' link points to http://patft.uspto.gov/netacgi/nph-Parser?...42574&RS=PN/6442574
which is US Patent #6,442,574, filed April 29, 1999 and issued August 28, 2002.

The abstracts are identical, but the details are different. The lists of references are almost identical, but #6,442,574 includes a reference to #5,933,841.

Then, under "Parent Case Text", #6,442,574 says:

This application is a continuation of U.S. application Ser. No. 08/649,271, filed May 17. 1996, U.S. Pat. No. 5,933,841, which application is hereby incorporated by reference herein.

Shortly after that point, I exceeded my capacity to read patent claims while engaging in sentience, but it sounds like May 17, 1996 may be the date to beat for prior art.