Telefunken "Flat Plate" and a current-production copy 
marketed by Groove Tubes
On the left are images of a Telefunken "flat plate", and on the right is the reproduction tube.  There are a couple of features that positively ID the Telefunken:
1)  The bottom of the envelope has a well-defined bluish-gray layer in the glass
2)  Molded in to the circular depression in the bottom of the tube is a diamond shape.  This diamond shape is oriented so that the bottom corner of the diamond (with respect to the numbers molded inside the diamond) points to the gap between pin 1 and pin 9.

Other features that may not be as definitive:
1)  The flange crimps on the Telefunken are small, and the material is bent toward the top or bottom, whichever is nearest.  The crimps on the GT are larger, and they are split, with half the material being bent up and half being bent down.

2)  The Telefunken plates' side vents are larger (more rectangular) and are located closer to the top and bottom of the plate.

3)  None of the Telefunken's top flange tabs are bent over, and as a result, the top spacer has pulled away from the plate.  The GT tubes top flange tabs are bent over, with the exception of the one to which the getter halo support is welded.