I started shooting the Hevi 7's back about 2009 and have enough for my lifetime. My 835 loves them. To answer your question, the early years lot numbers are the ones to look out for. I only shoot the 3 1/2's so here is what you need to look for
300324, 300325, 300326 were FANTASTIC (340 in most guns)! The 300525's were GREAT at around 320. Clark Bush did plenty of experimentation with the Hevi's so you may find more info if you type in his name or Hevi 7's in the search engine. Most of the quality Hevi shells were around 2009-2010 time frame. Good luck!