Actually there are set necks Epiphone Les Pauls that are also made in China. In fact, the latter batches are mostly made in China at Epiphone's plant in China.
Here are what the letters in front of the serial number means. More explanation of th serial number can be found at the FAQ section of www.epifriends.net
S = Samick, Korea
I = Saein, Korea
F = Fine Guitars, Korea
U = Unsung, Korea
SJ = Sae Jung, China (though we came across some SJ serial numbers on guitars that carried a sticker made in Korea)
BW = China
DW = China
DC = China
Quality wise, there doesn't seemed to be much difference. Some of the older Korean models tend to be heavier than the China made ones. Other than the fact that all China made models have Grover tuners, there are no other significant differences. In fact, I think the quality is improving as the Epiphone China plant (it's actually named Gibson) gets more experience in producing the Epiphone guitars.