I have a beta BHK 250 and it was updated a while back, though the only thing I know for sure was changed are the binding posts. The original binding posts had a bad habit of stripping the threads, not good for making solid connections to your speaker cables! While this is not definitive, the early binding posts had a hexagonal head, perfect for applying too much torque. The new binding posts are round and knurled, perfect for struggling to get them too tight. I’m pretty sure this change was made either before regular production began or shortly thereafter. With respect to the BHK 300, I not sure if early units used the hexagonal binding posts, but if they are there it’s an early build unit.
I can’t speak to the BHK Pre since mine is a recent build and I don’t recall any discussion of changes in the lead up to my getting it.