This MS4B is of Catkin construction, with a copper anode sealed directly to a glass envelope and then enamelled.
The MS4B was made in both catkin and conventional glass envelope versions, but the electrode system designed for the glass envelope version would not fit the catkin envelope. Thus the /K version was redesigned and became superior to the glass version.
The enhanced image shows the type designation on the screen.
The top of the screen contains an insulated plate to carry the top cap.
The base and B5 pins can be seen, the internal construction is obscured.
The MS4B was designed as a amplifier tetrode for RF and IF applications in broadcast radio receivers.
The MS4B (introduced 1931) was an improved version of the MS4 (notably, higher gm). The /K version is a classic Series 1 catkin dating from 1933-4. Series 2 catkins (1934) were taller, with B7 base caps and tapered external shields.
The Type VMS4/K (also Series 1) were externally identical to the MS4B/K, except for marking. Both types also exist without external shields but are rare in this form.
The screen over the copper-glass envelope is 33 mm in diameter and, excluding the B5 base pins, is 109 mm tall.
References: Datasheet, 1005, 1040 & 1043. Type MS4B was first introduced in 1931.