The 8298A was designed for mobile transmitters and the heater was designed for a wide range of input voltage and not just the 10% around the nominal 6.3 Volts. The Type is generally known by the alternative Type designation of 6146B and this exhibit carries both codes.
The GE logo and under the white printing can be seen the original etched lettering showing the 6146B and made in USA.
The working face of the anode has additional heat dissipating metal at the centre. This flange is conveniently the point where the two halves of the sheet anode have been joined.
The wide glass tube envelope is 38 mm in diameter and excluding the IO base pins, the valve is 80 mm.
References: Datasheet & 1040. Type 8298A was first introduced in 1964.