Mate, believe it or not Millwall where a bit flaky and not overly rated by the top firms, for simple reason they didn’t travel that well most of the time, yes they were a bunch of evil fukers and the old den was mental, but didn’t do Cardiff, United, Everton, Forest, Man City, Leeds..away from home...Boro did and if you know any of the top firms from back in the day? They will confirm this.
I've read umpteen hoolie books in my younger days (cringe I know) and Boro seemed very highly rated and probably the most organised out the lot of them