Those tarballs are made by checking out a pristine pike tree from CVS, and running "make snapshot_export". In your case, preferrably checking out with cvs -D <iso8601timestamp>, picking a suitable UTC time from one of the appropriate times when export.pike tagged a "stable" tree.
A fairly good (if perhaps slightly cumbersome) way of finding such UTC timestamps is to parse the JSON output from
http://pike.ida.liu.se/_/cvsview/commits/7.7/src/version.h?u=Pike&f=2007...
for the second commit to src/version.c with a commit message matching "release number bumped to * by export.pike", the * of which is the build number. Its "T" key is the <iso8601timestamp> timestamp in UTC. (Feel free to vary the &f= argument to later dates, as you move on, as one possible method for filtering for only new builds since last time you ran your script.)
Presently, that thus means the relevant checkin is
{ "dt":1, "module":"Pike", "repository":"Pike", "committer":"peter", "t":1181472961, "T":"2007-06-10 10:56:01", "message":"release number bumped to 33 by export.pike", "files":[{ "add":2, "rem":2, "tot":15, "change":"edit", "ancestor":"1.390", "revision":"1.391", "path":"7.7/src/version.h", "dl":"http://pike.ida.liu.se/cvs/Pike/7.7/src/version.h?1.391", "show":"http://pike.ida.liu.se/development/cvs/view.xml?module=Pike&file=7.7/src...", "diff":"http://pike.ida.liu.se/development/cvs/diff.xml?module=Pike&file=7.7/src..." }]
and its timestamp thus 2007-06-10 10:56:01 UTC.