Rectifier question

I would suspect it was a bad reg/rec.

FWIW, when I put the GL500 engine into Eccles I noticed that the wires on the original reg/rec were getting stiff and I had the one from the bike the "new" engine came from so I swapped it in. About a year later it showed signs of the battery not holding charge and when I checked I found only 12V at the battery at several thousand RPM. I swapped in another good used reg/rec and it has been fine since.

BTW: Mike is right about the burned connector.
When the terminals in the connector are new the internal resistance will be close enough to zero to make no difference but over time exposure to the air will cause the terminals inside any connector to oxidize. Eventually the oxidation will work its way between the contact points and the resistance can increase to a measurable level. When that happens 2 electrical principles come into play:
1) The voltage divider principle, which states that when there are multiple loads in series the voltage across each of them is in proportion to their individual resistances.
2) Ohm's law for power states that Power = voltage multiplied by current (usually expressed P=E X I and measured in watts) and any power developed in a resistance has to be dissipated as heat (more power = more heat)

When the resistance is close to zero not much voltage will drop across it but as the resistance in the contact points increases the voltage across them will increase and as the voltage increases the power will also increase, causing the terminals to heat up, which will accelerate the rate of oxidization, causing the resistance to increase which increases the voltage, power, heat and so on until they become hot enough to melt the plastic.

So what causes the oxidization? Exposure to the elements (= rain water and dirt finding their way in) can play a role but mostly it is just exposure to the air for a few decades.

And how can we prevent it? The manuals never mention it but the terminals inside all of those connectors used on your bike really should receive periodic maintenance. The easiest thing to do is to simply unplug the connector and plug it back in again every 4 or 5 years so that any oxidation between the contact points is pushed out of the way.

The connector can also overheat if the terminals are loose and not contacting each other solidly. This can be fixed by gently squeezing the female terminals to tighten their grip.