Help on Telefunken tubes

Telefunken 12AU7 and 12AX7 come in both smooth and ribbed plates. I just looked at several samples of each from my collection and the plate length and overall construction appears to be almost identical between these two Telefunken tube types.

The only way I can see is by the pin out to the ring getter. To check this, turn both tubes upside down, and look for the gap in the (otherwise) solid row of pins.

With this gap turned to 12:00 O'clock position, if the first pin CLOCKWISE supplies the getter, this is a 12AX7.

If the ring getter is connected a few pins further clockwise, this is a 12AU7. Hope this helps.

Albert is right, but I may have an easier way. In good light, exhale onto the surface of the tube, the patterns of the long gone paint often become apparent enough to read in the condenstion of your breath.
Marty

Marty- terrific hint. many thanks.

Thanks Albert for the advice. The tele in question turned out to be a au7. Good listening!