The YD1050 is a planar triode with attached aluminium cooling fins for forced air cooling. It is designed to operate at up to 2.5 GHz.
The side view. The valve would be operated in a co-axial cavity. The ceramic insulation suggests high voltage and temperature. The ceramic section is 19 mm diameter.
The heater is connected to the two inner rings.
Diagram of connections. Image courtesy of Radiomuseum.org.
The envelope is 32 mm in diameter, and overall is 67 mm tall.