For rust to form in the area you've reffered to, water had to have collected and allowed to remain to attack the metallic surface of the spark plug.
That portion of the plug is plated, either nickel or chrome.
If it is from water, you would deduct that the ellavated temperature of the plug's external surface would evaporate what fluid collected there.
Did the surface contamination readily wipe off?
Were the seat and threads free from the "rust"?
Could this be a combustion leak?
Try again and use an anti-seize on the threads and the seat to see what happens on the new plugs.
There is a torque spec but I can never find it so I tighten to an educated feel.
I've heard screw in till contact in the seat or against the gasket, then 1/4(90 degrees) turn tighter.
Happy Trails Beep Beep from Houston!!!
Donald, El Cheapo

Hi Donald - the seat and threads were absolutely free from rust. It had rust just where the porcelain sits. Not much of rust, just rusted enough to know its rust. As for the other details, I cannot really remember well as the old plugs were discarded. I might just check on it on and off. I had the engine overhauled last year - and its serviced according to manual. I dont think its a leak in the combustion ... or should I perform a test to confirm?

__________________
... Kerry

126 tailed by a 203, 129 leading the pack.

The rust colored stuff, was it red as you think rust should be?
Was it at the interface of the insulater and the metallic body?
Spark plugs can have combustion leak at that joint between the insulater and the body.
As mentioned before, try this set and follow the directions given by all, see what occers.
Happy Tails Beep Beep from Houston!!!
Donald, El Cheapo always trying

It is the color of what rust ought to be, and not red.

I'd be standing by this post and watching the parade go by.

Should I stand at ease, or utmost attention?

__________________
... Kerry

126 tailed by a 203, 129 leading the pack.

I guess I missed the boat, for I see no parade.
I wanted you to more isolate the area of the rust, or was it a combustion gas condensing on the cooler insulater.
Happy Tails Beep Beep from Houston!!!
Donald, El Cheapo, the last man in the paraide

Hi Donald - I referred to the parade of views coming through, and was wondering whether I ought to be more concerned, or its a small matter to worry about.

I just got off the phone with the mechanic who changed the plugs and he reckons it could have been water vapour from the petrol as he had a similar client with the same complain.

He did say its nothing to worry about.

I certainly hope so.

In any event, I shall be keeping 2 eyes on the plugs in 3 months' time when the next service interval is due. Meanwhile, the 300se runs as smooth as ever.

__________________
... Kerry

126 tailed by a 203, 129 leading the pack.

Heed to what your mechanic says not what we guessers come up with.
I keep two sets of plugs on hand, one in the engine and the other on a place board with six holes to hold the plugs in the cylinder position removed from.
I swap the sets every 7,500 miles to see the cylinder performance. I have nothing more important to do, so why not keep up on the engines condition.
You're going to be just fine.
Don't let anyone steal your JOY.
Happy Trails Beep Beep from Houston!!!
Donald, El Cheapo