The BCP-15 was an original Freed design. It was originally spec'd at 50 madc and 40 Henries. We built many of these to the original specs. These would tend to have only the BCP-15 marked on them.
Later... realizing that most people who were purchasing these plate chokes were using them with the type 45 tube which runs moreso typically at 35 to 40 mils of plate current. After careful analysis of the original design we were able to regap the unit and optimize it for use with the 45 tube at the ballpark 40 mils instead of 50 mils of plate current. With the smaller airgap, we picked up more L (50 henries).
Once we offered the second choice -- 40 mils at 50 henries or 50 mils at 40 henries -- I've tried to be consistent (bearing in mind the existence of human frailty) and mark the coils that were gapped for 40 mils. Often times (but not always) this demarcation might be written on the back side of the coil.
The 50 mil coils may or may not have the marking -- if it's not marked--- again allowing for the occasional human error -- it is most likely gapped for 50 mils.
This description comes from the Bottlehead forum for "All things MagneQuest." Check out the thread here.