There are more than one st350 tweater, I believe, the older type, alnico magnet from the frount look like st(?) 35 or k77 but with larger magnet, and superior sound, and the st350a which uses a mud magnet and still sounds pretty good and I believe more efficient to the k77. I use both the k77 in my cornwalls and klipschorns, and the st350a in a diy horn speaker and the st350a tweets really shine.
IMHO
Shane
If they are the older larger 350 they may be well worth the price.