[TimG]

Stefano,
The factory regrinds are very interesting and there are a number of articles published by Al Greening on the subject.
When the assembly date/code was stamped on the pad at the engine assembly factory, often times there were errors made by the person hitting the gang.
Sometimes as in late 1967, they took non Corvette motors and converted them to 327/300 HP Corvette motors. The person that made the "hit" also had a powerful hand grinder available to quickly grind the surface clean and correct the error that was made or make a necessary change. This is still being done today on the engine assembly line. Speculation was that a worker may have pulled the wrong gang and incorrectly stamped a motor with the wrong code. It needed to be corrected and the procedure was to take the powerful hand grinder and within 10 seconds clean the numbers off the pad. Of course, this wipes out the broach marks and the resuting marks run most of the length of the pad. As the ID number of the car has not been hammered on the pad at this point, only part of the pad is ground. These guys did this many times in one day according to most sources and they got good at it. The traits are similar on factory regrinds, it is just hard to prove that a particular grind was done at the factory. Fortunately, my car is a very original car with the security/warranty gaskets still in place on the engine (these gaskets were only used on the assembly line and are not reproduced, this was the way Chevrolet determined if an engine had been tampered with) It becomes tough when a fuel injected car is restored and it shows up at an event with a reground pad.
Many times it is not given full points. In many cases people shy away from a pad that has been ground and restamped at the factory. With the master pad library, it is possible to compare a stamp on an engine with a stamp on an engine in a similar range to see if the stamps are similar. When I found my car, I was hesitant to buy it because of the grind, but after inspecting the car I thought it may be an asset as it had to happen at the factory. There were many master judges and photographers looking at the pad all weekend. Some may say it's silly although I really enjoy these unusual things about cars.
And yes, some casting numbers, dates are done with epoxy and these do not last long when a block is cleaned. What a mess.