In the thread titled Skylake 6700K/6600K Batch and Serial Numbers, US-based forum member xxbassplayerxx keeps track of the overclocking results posted by various members of the community. He updates a publicly available Google Docs regularly detailing the overclocking capabilities and characteristics of the Core i5 6600K and Core i7 6700K processors.
In total the document contains 125 reported overclocks from 55 different batches. The oldest reported batch is an engineering sample Core i5 6600K from week 10 this year, L510C971. The newest batch is from packaging week 38, L538B747. The data contains mostly information of ambient cooling overclocking, but once in a while you will find liquid nitrogen overclocking results included as well.
If you're interested in finding out more, go have a look at the forum thread here. If you like the effort xxbassplayerxx puts in maintaining the data, make sure to give him a thanks too!