No, it isn't. The Lava is a recast of the PP2, possibly taken from a repaired mold that Rogue Studio original plug came from. The Lava helmet and mystery helmets have no ties that we can see to the ESB or stunt helmets, aside from our belief that ALL of the original Hero helmets, ESB included, came from ONE mold.
I would argue stating that "there are 4 known casts from LFL in private hands" as I think it is a bit misleading, although I know what you are trying to say.
To the best of our knowledge there is ONE hero helmet in private hands; the Prepro 2. The Prepro 2 was cast back in the early 80s and the result was the plug that Rogue Studios now owns. The mold that created Rogue Studios mold was heavily damaged while taking the plug out and was THOUGHT to be discarded. However, it would seem that either the Prepro 2 was cast a second time OR (and I think this is the more plausible explanation) another plug or helmet was taken from that damaged mold which eventually resulted in the Lava helmet (don't know how many generations the lava helmet is from that original mold) and well as the mystery helmet(s) and most likely the Don post helmets.
We know that there were originally 6 hero helmets and we more or less have them all accounted for. However, we don't know how many stunt helmets were made. We know of the two in the archives. We believe the helmet Max Cervantes got (with almost the EXACT same story as Banzai shared with us) is a stunt, as I believe Banzai's helmet is a stunt as well. Are Max's and Banzai's helmet's brothers? They share some but not all the same feautres. Banzai's looks closer to the stunts than Max's. Did Max's and Banzai's come out of the same mold as the stunts in the archives or were they cast off one of the stunts in the archives or were they cast off a stunt we have never seen... We don't know.
This other helmet we have talked about is something that we really can't talk a lot about and quite honestly, at the moment, I am dismissing it entirely. It does lack the top reangefinder ear platform like Banzai's but that is about all the similarities that they share. Unless the owner of that helmet cared to share more... I am not even throwing it in the mix.
That is about all I can think of right now.