Spare tube holder in Mark III?

[robredo_mattila]

I have a mark 3 simulclass purplestripe short head with no reverb. Looking at the bottom of the chassis I can see FOUR 12ax7 preamp tubes, and there seems to be a Fifth 12ax7 [or similar] tube inside a metal casing next to one of the other tubes. Is it a spare tube being held in place? is it even being used? Why would Mesa put a tube protector over one preamp tube and not all 5?

 

[Boogiebabies]

That's the V1. It's shielded to precent any noise being passed down to the next eight preamp tube stages.