In an unpublished letter to Bob Fabris (dated November 1, 1981), Mark Keller 
wrote:

Dear Bob,

Here are the answers to your problems with my programs (I hope).  Lines 30-70 
of Blockout 5.0 are:

50 CX=60;PRINT #2,L,
70 BOX C,-43,K,2,3;BOX Y,X,2,2,3

Yes, the line sequence in Pinball I goes 9, 10, 20, 40.

Both games ["Blockout" and "Pinball I"] were written as short, simple (and 
unfortunately slow) graphics games.  

Blockout 5.0 is played like Breakout or Brickyard.  The game will prompt for 
parameters and hand controller #1 moves the paddle.

"Pinball I" is a simple pinball game (no gravity).  It will prompt for 
parameters, triggers 1 & 2 control the flippers.  They stay on a set period of 
time when used and then are disabled for a set period of time.  I'll send more 
game soon (I hope), and am now working on a Bally -> Vic-20 interface.

Mark