[Arran]

I guess you mean the second detector or AVC diodes, it seems to have been a matter of preference by the manufacturer, or perhaps the engineer, what to do with the second diode, if they chose not to use both of them. I've seen some where both plates were tied together as well as tying the plate to ground, I don't think that one approach won out more then the other. In fact I have some radios where they used a 6H6 double diode, rather then the pair in a 6Q7, and they tied both diodes together, other then for tube stuffing the chassis why even bother?
Regards
Arran