The ECF80 was designed for TV mixer operation up to 220 MHz. As 1950s television receivers tended to use the PCFnn 300 mA valves, the production of this valve with a 6.3 Volt heater suggests that it was aimed at the professional VHF communications market.
RFT was the state owned East German valve maker.
The triode is the oscillator and has to produce 5 V pk-pk signal. The screened pentode is the mixer. The valve has two cathodes so two independent valves are sited in the one envelope.
The small triode section is on the left.
The triode anode with pentode anode behind. This side of the pentode anode is left bright - overheating the triode not being a good idea.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 49 mm tall.
References: Data-sheet & 1040 Type ECF80 was first introduced in 1954.