The "K" serials are Custom Shop ESP's. The pre EII, ESP Standards, were not Custom Shop guitars. IMO the pre EII, ESP Standards, were much higher quality/better guitars than the EII's though.
I have a few ESP Standards that are as good of better than many ESP Custom Shop models I have owned. The EII's I have tried, not so much.
So are k serials what to look for? I'm a lefty and found a Kirk sig ESP custom shop with k as first serial letter. Good price so I'm thinkin of getting