I was for a time ago reading about some interesting old Burley's. And BURLEY#21 was one of them so I will post a answer sent from ARS-GRIN (Jessica thanks):
"Hi Hasse Sorry it has taken me a while to get around to responding but we have been very busy planting the last couple weeks. Burley 21 is considered "high nicotine". There are also low intermediate (LI) and high intermediate (HI) lines that fall between these. Many years ago they found that alkaloid production seemed to be controlled by two major genetic factors (they called them nic1 and nic2). LA burley 21 is recessive (nic1, nic2) and Burley 21 is dominant (NIC1, NIC2), with the intermediate lines being (nic1, NIC2 = "HI" or "MA") and (NIC1, nic2 = "LI" or "LMA")."
-I hope someone think this "thing" is interesting..