The MX420 is a high voltage rectifier for use in medical X-Ray equipment. Sadly we can find no operating details, but typically medical X-Ray machines run at 40-150 kV and up to 400 mA.
Typical of high voltage devices the envelope has been pumped to a very hard vacuum without gettering. The filament looks to be tungsten.
The reentrant top of the envelope provides a significant amount of glass to hold the anode. The connection is most substantial and would also act as a way to conduct heat away from the rectifier.
The filament is a pair of circles bent into loops forming four emitting surfaces. The wire is thick indicating a high current as well as a high voltage application.
The filament supports from below.
A basic X-Ray image of the MX420. The filament wires are on the left and the substantial anode seal glass is on the right.
A different exposure to the X-Ray beam reveals more detail. The filament connections are particularly clear within the Ediswan Screw base. The form of the filament is also made very visible in this image.
The balloon envelope is 120 mm in diameter, and including the base pins is 282 mm tall.