The SP41 is an amplifier pentode with external metallisation as shielding. The original design was from Mazda, and other manufacturers copied this successful design after it had become established.
The SP41 and SP61 differ only in heater voltage rating. They resulted from Mazda's strong design initiative during the later 1930s.
The demand for efficient video pentodes for TV came a year or two earlier in Britain than elsewhere and the SP41 was Mazda's solution. Shorter leads to shorter pins, and compactness in general, are essentials where high gain-bandwidth product is a requirement and it is easy to see why Mazda's SP41 was streets ahead of (say) Mullard's SP4. Of course, the later EF50 overleapt the SP41 but the latter and its 6.3V lookalike filled vital needs in military radar and high-frequency communications during the late 1930s and early 1940s, until UK (and later US) production of EF50s was sufficient to meet demands. Moreover, one reason for the huge demand for EF50s was that they had short lives and needed frequent replacement. By contrast, the SP41/61 were virtually indestructible.
The 4.0 V heater was common in the 1930s when mains sets first began to appear.
The classic envelope is 30 mm in diameter and, excluding the MO base pins, is 79 mm tall.
References: Data-sheet & 1040. Type SP41 was first introduced in 1938.