I have not tried this tube, but I believe the RFT is not Telefunken. It IS a RFT, which is another brand all together.
There are many copies of Telefunken, some made in East Germany that are similar but do not offer the same performance.
A few years ago there were a number of posts about a German tube called the RSD brand. I bought several on the recommendation of a tube dealer and having read glowing reviews in posts like Audiogon.
In the end, after hearing the lackluster performance (among the ones that did not fail promptly) I disposed of them in the trash can rather than pass them on to another member.
If you can afford real Telefunken 6DJ8 (the tube you need), go for it. If you can't find it or refuse to pay the high price, try a Mullard 6922, Amperex 6922 or 7308.
Should you run across a Siemens 6DJ8 or 6922, it too is a gem. Good luck with the tube rolling.