They are GTB type tubes, they look of the same construction.
IMHO there is nothing to be concerned about, just enjoy them.
I have to say that Im not alone if I say the TS 6f8g RP sounds better than their 6sn7 counterpart?
I have both their round and oval mica of the 6f8g and its sounds better to me than their 6sn7 RP oval mica.
Even have a rare 6f8g square mica from TS that havent tried yet.
They sound very similar but the round mica sounds better to me. Just a tad more musical. I could live with either one but if i had to choose I would take the round mica. Have not tried the 6sn7 round mica though.
There is also a Ken-Rad 6F8G with oval plates worth trying.
The TS 6F8G with the square mica have flat plates, and are not so rare in my opinion.
The 6SN7 with square mica on the other hand is very rare.
As a matter of fact, the code also tells us whether that valve is a GZ33 (rS) or a GZ37 (IS).