Look at your AR. There are two locations, one in the upper left (furthest from the heatsink, near the connectors), and another below the 8-legged LM305 tin can-looking part, where on the AR that produces 6V, you will see diodes in those two locations. On the other AR that makes 5V, those two spots likely have resistors in them.
The AR-I-01's used on Asteroids came with diodes from the factory. But some operators changed them BITD to resistors, so the voltage will measure more accurately with no load. The tradeoff is that if either the 5V or GND lines get interrupted or broken between the AR and the game board (e.g., by a dirty edge connector, or having it not on all the way), those resistors will burn up (which is what happens on the later-model AR's, which all have resistors instead of diodes in those two spots). You can replace the resistors, as they are just 10 ohm 1/4 watt ones, but when you have the diodes in there, they will not burn up if one of the lines is broken.
In the end, it doesn't matter which you have in there, as long as you know that the voltage will be artificially high if you have the diodes.
And yes, I repair all Atari vector game boards and monitors. PM me anytime for info.