The E2521 is an experimental triode from M-OV. It has a frame grid that connects to five of the nine base pins. This was designed for UHF use in the grounded grid configuration. The GEC A2521 has the same internal construction and the same pin-out and is most likely the production version of this prototype.
The anode is black and near the base. The working face is small and projects up from the bulk metal heat-sink. The grid is the main structure. The frame structure is fixed to the side supports. The rectangular cathode tube is coated on one side only.
The vertical supports are precision cut mica. The frame for the grid looks to be gold plated.
The cathode is connected to the base pin with metal tape.
The frame grid. Frame grids are wound at a tension of around 50% of the breaking strain of the wire to ensure that the wires lay flat. The grids are wound and thus start out as double sided and wire can be seen to pass over the edge of the frame. Once wound and the wire secured in place, the underside is trimmed to remove the lower wires. In this micrograph the cathode is at the top and the bright face of the anode can be seen through the grid.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 48 mm tall.
Reference: Observation and A2521 data-sheet. Type E2521 was first introduced in 1957. See also1957 adverts.