Updated October 2008:
'Gold Seal' engines are factory replacement complete engines painted gold instead of the usual maroon or black. They are remanufactured engines i.e. ones that have been exchanged under warranty or for some other reason. They will have been completely dismantled and every component tested and measured to make sure they meet spec, and any that don't are replaced. As such it is probably better than an original engine, which are simply assembled from parts out of a box with the quality and 'correctness' of those parts assumed. I don't know whether replacement parts in a Gold Seal engine were similarly tested, but even if they weren't at least the original parts have been! My roadster came to me with a Gold Seal engine of the correct type for its year, so either it was replaced under warranty or a PO had been very particular about fitting the correct replacement. There were also Silver Seal short engines, i.e. without heads or ancillaries. Gold Seal engines have special prefixes which can be decoded to the original prefixes, see below: