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.
For builds by pikefarm this is indeed true.
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.)
For builds created by make export it's better to use the tag generated at export. eg
cvs update -r v7_7_33