The 6J1 is a Chinese amplifier pentode. It arrived as part of the FU-32 SE amplifier kit where it is used strapped as a triode. The presence of two cathode pins suggests that the design was originally intended for RF use and as a near equivalent is the EF95 this is indeed the case.
Internally the connections from the base pins are copper. The box anode is open at the sides and thus the three grids are visible.
The working face of the anode.
The heater is folded so that it passes ten times through the cathode tube.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 38 mm tall.