We know nothing concrete to my knowledge; only rumor, unless you can shed some heretofore unknown information. The rumor has always been that the Hunter brothers would make and sell guns at lesser prices to special friends/persons, or trade guns for favors, or dogs (John Hunter specifically). To date I've never seen, or heard of any materials documenting such behavior; and the best evidence I've seen that such may have taken place is found in the original pre-1913 shipping records. I've personally handled two A-2 12-bores recorded in the ledgers as a PE Grade; and one A-3 recorded as an A-2. The story I've always heard is that a Hunter brother would commission a special order gun, personally manage it's construction and final inspection; then record the gun as a lower grade, for price discounting purposes, within the shipping ledgers. Who knows if that is rumor or truth?
We do know that Hunter made some special guns graded "A-4" because Kraus lists the A-4 Grade on his engraving schedule; BUT Dr. Stubbendieck's found no listing for any A-4 Grade guns in his exhaustive research of the original ledgers, so we've no idea what constitutes an A-4 Grade. John Houchins theorized in his book that an ungraded Smith gun I once owned was an A-4, but that gun is recorded in the ledgers as a "Special". Hope this helps, but in the interim we remain all ears should you have access to any hard documentation.