My theory is not so much the burn-in cycle. I suspect that a really flat grinded brake disc has a good chance of always staying flat. But a brake disc with the smallest waviness can become exponentially wavier within a short time. Obviously, the very narrow wheels of the 4/4 can amplify this effect? Maybe because they run less stable round as it is the case with a wider tyre?
Another guess...the brake discs on other Trad models were, I heard, bought in, representing a reliable industrial standard.. Morgan cast the brake discs of the 4/4 themselves for quite a while (so I was told, I don't know). But they weren't really straight in the first place, so grinding them down before first use makes a lot of sense.
