The valve shown is an MKT4/7, ie fitted with a B7 basecap. Most MKT4/5's were fitted with B5 base caps with the screen grid connected to a side terminal.

The MKT4 shown in the photo is a late (c. 1950) specimen and has a distinct Cossor look about it. It is probably a Cossor MP/Pen re-labeled as an M-OV valve.

In single ended audio configuration would provide an adequate 2.5 Watts.

Some pre-war EMI television receivers used the MKT4/7 as a line output valve.

It was a design that was produced by several different manufacturers. Note also that MP-Pen, MPPen, MP/Pen are variants describing the same type of valve.

The classic envelope is 53 mm in diameter, and excluding the B7 base pins, is 110 mm tall.

References: Datasheet, ¹⁰⁴³ & ¹⁰⁴⁰. Type MKT4/7 was first introduced^¶ in 1937.

 
Pin Connections

B7	1	2	3	4	5	6	7
	-	g1	g2	h	h	k	a

 
Absolute Maximum Operating Conditions^¶

Beam Tetrode	Vh	Ah	Va	Vs	Vg	mAa	mAs	gm	Pout	D
	4.0	1.0	250	225	-13.5	32	5	3	2.5W	10%

Thanks to Frank Philipse for supplying the above PDF datasheet

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