According to the photography, this is "one of two" in existence. By the time Rocket-Firing Boba Fetts were being finalized, they were inches from being released to the public. It was at the final stages of design/release.
If this "Holiday Special Fett" is only 1 of 2 examples it wouldn't have a date stamp. This early on, we would be looking at a hardcopy, painted hardcopy, or first shot. None of these early-phase development pieces would have date stamping.
Although . . . I'm glad you're using the archive for reference!
R. Jason Coulston