Reading (between the lines of) Mark McGrew's post from the past, he expresses concern milling/machining will not be able to make an exact duplicate of the thin lip's contours seen on early covers. His son said, "Why don"t you just hand grind these things as there will be some hand work involved at end of machining anyway?"
What Mark's son is suggesting (my interpretation of his comment) is: Instead of machining off the material to make a thick lip cover a thin lip, hand grind the thick lip to a thin lip.
In other words, after a late cover is machined to have a thin lip, hand work will still be required to give the original appearance of a thin lip.
Machining the thin lip to the exact appearance of original may likely not be possible, because the cutting tool will not have the contour of the radius/radii of the thin lip to exactly produce original thin lip appearance.
In my limited first hand machining knowledge/experience, the ability to re-produce the exact contour of the lip, the contour is created by the shape of the cutting tool available for making the cuts. The ability to make the lip contour exactly like original is limited by the shape of the cutting tool available to be used to make the cut. If I'm reading things right, Keith Byers expresses a similar concern/consideration in his post.
My E254 engine did not have an early cover. I removed the material by hand, but only on the upper portion in the visible 10 o'clock area. I considered removing the material on the rest of the cover's periphery, but did not, in hopes I might find a thin lip cover........ some day. These covers are rare, as we all know. Probably, at some future date, as time permits, I'll wind up removing my cover to remove by hand the material of the thick lip to make it look like a thin lip.