One for the Mesa amp geeks this (of which I am one!)
The internet wisdom states that there was an early Mesa .50 caliber, which came with no lead master and metal switches, like this. It is often criticised as the channels were hard to match on volume against one another.
Then there was an updated .50 caliber plus version, which had an additional control (lead master for matching the channels), plastic switches and 6L6's rather than EL84s. This is more sought after as it fixes the channel volume issue.
But, there was also an interim version. Which had EL84s, metal switches and the lead master. So you can have the same thing with EL84 goodness. Who knew!