sharing the same luck of flying the Tu-134 in a couple of weeks too.
From Dmitry Komissarov's magnificent "Tu-134 - Tthe USSR
's Short-Range Jet Airliner", I'll try to answer your question.
The first two prototypes, based on the Tu-124, carried CCCP-45xxx regos. Then, the production Tu-134 (and the first prototype Tu-134A's, I think) were mostly 656xx. That went from 1966 'til 1971.
And from that point on, it seems to be nothing more than pure chaos. For example, c/n 63700 (Tu-134AK) mfd 3/4/1981 became 65099, c/n 63731 (Tu-134A) mfd 8/4/1981 became 65901, and so on. My guess is, that they were trying to confuse spies (who were egaer to copy a 49-ton regional airliner
My guess is that the reason so many ex-Interflug/CSA/Malev aircraft became 656xx is because the 656xx was used on the very first Tu-134's manufactured in the 1960's. By the time the export aircraft were being re-imported, most of the 656xx-regged planes had already been decommissioned, thus creating a vacant range of registrations.
Hope this helps.
And enjoy that old beast.
EDIT: Sorry, while writing this post, I didn't really see BOAC911 had already answered your question.
[Edited 2006-08-03 18:26:57]