This prototype which appears to date from around November of
1981 is a late WIP version of the game. The gameplay is
all accounted for, but there are some different sound effects
and colors that were tweaked before the game was
finalized. Most jarring of these is the 'bong' sound when
the game starts, which was wisely changed in the final.
There's also a bug in this version which makes it impossible to
shoot an enemy at the very top of the screen (your shot goes
underneath them). Thankfully this was fixed in the final.