You're welcome!

B,C, D series were all listed as having a 40 Hz Fs, but never measured any, so don't know how accurate this is. I just know all the basic factory cabs recommended for them were tuned to around 42 Hz and since they tuned to Fs back then...........

Qts will be low to couple to a high output impedance and Vas will be high to keep efficiency up, so about the same as the original 'E' except for a published ~25 Hz Fs that in reality averaged 29.5 Hz in all the measured specs I've seen; whereas the 8G, 8H, 8K, 8L averaged 26.6 Hz. Based on these, the B, C, D, series may indeed have been closer to 50 Hz, but then what really matters is how much does cab, playback loading drops it.

No clue about re-coning, that's for GPA to answer, but I'm guessing that there's only one re-cone kit available for 604s these days and I'm betting it's not going to be the one listed for any older than the E and maybe not even its original. For sure, there's no early 515, 604, 803, etc. series paper surround cone kits left and why good condition originals go for major 'coin of the realm'.

GM