I shot Thunderheads for many years, (45 years of bowhunting), but frankly, considering the options today, they are a pizz-poor option... tuning them for good flight was a pain... these days there are many fixed blade heads that fly pretty much exactly like field points from a properly tuned bow... hard pass on Thunderheads.
Great heads. Still very effective, but your bow needs to be tuned well.
I'm definitely a NAP fan and loved my Thunderheads back in the day, but as mentioned above, bow has to be tuned really well.
Thanks for the heads up, I do not shoot them any more but I collect them. I love old bow hunting stuff.