I just emailed another tube dealer at Allegrosound.com who sells both WE 300B tubes with and without serial numbers. The ones with no serial numbers are cheaper. According to Rick at Allegrosound.com, there are no sonic differences.
My feeling is that the tubes with no serial numbers were not tested by WE but if they were tested by another company and tests OK, I assume the tubes are fine.
But it's just my guess. I think WE sold a bunch of non serial numbered tubes for economical reasons, which would be fine as long as most of them are legitimate.
Again, this is just my educated guess.