It looks like a PX4 but is marked A580 and Spec 1305. The Type designation is clearly within an M-OV etched lozenge on the envelope.
When Anthony tested the valve, starting with the PX4 operating conditions, he found that he needed a more negative bias on the grid to control the anode current. So a higher gm version of the PX4? or possible a modification for an RF application.
The front view looks exactly the same as the PX4.
The Type designation.
The markings on the glass.
Thanks to Anthony for sending these images.
The classic envelope is mm in diameter, and excluding the B4 base pins is mm tall.
