Perhaps you might give me guidance on this one question. It is:

Specifically what, other than 1) tags, 2) keys, and 3) collation sequence, are documented factors in revealed SYS(3054) showplans' differences?

We are still unsuccessful in solving this issue.

1) LIST STATUS indicates that, in both instances, all of the tags and their collation sequences are identical.
2) Both .DBCs, "development" and copy of "production" data, are on the same server.
3) Both have close to the same number of records.
4) The SELECT - SQL expression ran adequately for years and is unchanged.
5) The "development" data completes in approximately 0.015 seconds 
   (as did the production data until last week).
6) The "production" data completes in approximately 23.6 seconds

Thank you for all of the kind help you have given.

Spencer