yep, stated already, but per brasington.org -
Game is in cocktail mode, how do I change this?
Centipede and Millipede do not have a cocktail DIP switch like some games do. Instead, Atari used a pin on the harness to determine upright versus cocktail mode. For millipede, this is pin 16. Millipede pin 16 should be left open (high) for upright, or grounded (low) for cocktail. This is backwards from how centipede does it. So if you are using some sort of centipede to millipede adapter or have rewired your harness, check that pin 16 is properly setup.
To assist, the multipede testmode will display the text "UPRIGHT" or "COCKTAIL" to show you what the current mode of the game is.
Multipede does not change how upright versus cocktail is selected, it continues to be determined the same way the millipede hardware determines it.
If you are using the arcadeshop.com centi/milli adapter, please make note that for upright move you need to remove the cocktail/upright jumper on the adapter.