Is a 6th axis possible?

How many axes are actually supported by the PRSalpha controller & SB3? Everywhere in the documentation, 5 axes (6 drivers, since two are used for the X axis) is mentioned as the maximum. But my control box has a mounting position for a 7th driver (it came with 6 installed since I have dual Z and an indexer) and a connector on the board obviously intended for plugging it into (it's right above, and identical to, the "DRV5" and "DRV6" connectors used for the A & B drivers. And, it is labeled "DRV7"). If I installed a compatible driver and set the number of axes to 6, would that 6th axis actually function correctly? Would all the commands that reference the number of axes simply work if, say, I entered M6 (Move 6 Dimensions) instead of M5? I could just try it, of course—except that those ARD-A drivers are VERY expensive and I wouldn't want to buy another if I couldn't use it.

There are lots of experimental gadgets that I can imagine attaching to a 6th axis (called C?) if it were available.

One potential issue is that the System Variables only include space for 5 axes (for example, Driver Channel 1-5 are SYS VAR #36-40, and SYS VAR #41 is used for something else). The values for a 6th axis could be stuck in high-numbered variables beyond those currently used, but that would be clunky.

One possible explanation for the presence of a 7th driver position but the apparent absence of software support for a 6th axis is that ShopBot is allowing for the use of another axis that, like the X axis, requires two motors and therefore two drivers.

Anyway, it occurs to me that I might be able to test all this by unplugging the cable from my A or B driver (DRV5, DRV6) and plugging it into that DRV7 connector instead (then making all of the appropriate software settings changes). But, I don't know if SB3 detects an axis driver that is specified but not connected, so it might choke if I say I have 6 axes but it didn't see the A or B driver.

I'd appreciate a definitive answer on the possibility of a 6th axis if anyone has one.