The Fine Things are Always Hand Made
That is a new one for me. I posted your inquiry on Antiqueradios.com. There are some good tube experts there that do not participate in this forum.
Thanks Norman, I am very curious about this one....
Is the tube inside the box also labeled 6J5G-RC? One suggestion I received is that it may be an RCA tube (possibly Canadian) boxed and sold by Sylvania.
Can you compare the internal construction to a non-RC labeled Sylvania tube?
Can you take the tube from the box and show it here, alot will be determined by that as to any conjecture on alternate manufacturers with a Sylvania rebrand.
Of this same era, I have a white ceramic based Sylvania 6J5G tube which was labeled 6J5GX. The base is original and has the Sylvania name along with the Sylvania "leaf".
Sorry for the delay...I removed the tube from it's carton and compared it to another nos sylvania leaf 6j5g...The only difference is the date code and that is has the words "for replacement use only" stamped onto the base along with the sylvania green leaf logo. The date code looks unusual too "2ZR" usually the year is the 2nd digit on my other sylvania tubes. (could be an RCA date code?)The internal construction is identical to my other NOS tube. But I do have others that are in my set with 1941 date codes that have a different plate. Also I noticed that the number on the tube has a period next to the G where the others do not...There is no "RC" stamped on the tube anywhere. Below are some pics, but they did not turn out to well...thanks again for the assist....
The "for replacement use only" was used on WW2 manufactured tubes. The idea was that they could only be used in existing radios and not new equipment.
Date code looks like May - July 1942, with the numbers being backwards from the way they were normally printed. The R in the date code is probably for replacement.
That is a Sylvania made, Sylvania branded tube, 6J5G tube- nothing unique at first glance. It is a wartime manufactured tube as suggested. To my understanding of the Sylvania date codes of that era, it was made in the last 2 week period of 1942.
Thanks Igor, must be one of the last before they changed to the lightning / triangle logo