This looks to be an experimental frame grid triode. The pin-out was arrived at by inspection. The heater seems to be 2.4 Volts at which voltage the current is 0.2 A. Painted on the envelope are the numbers 1.34 but it is not clear what meaning they have.
Looking at this exhibit and the A2913 the construction is similar and this may have been a prototype.
This image is taken looking directly at the back of the cathode. This can be seen to not be coated on the reverse side. The frame grid is seen face-on but the wire cannot be seen as it is so fine. The wire is in fact wound round the outside of the former.
With a cheap USB microscope the winding on the frame grid becomes visible. The view is with the rear of the cathode tube in the centre. The cathode is approximately 2 mm wide.
The thin glass tube envelope is 17 mm in diameter, and excluding the B7G base pins is 46 mm tall.
