The 7L7 is a sharp cut-off pentode with a high value of mutual conductance. The design use is within FM radio receivers as an IF amplifier and/or limiting amplifier to reject AM.
The outer screen obscures the inner electrodes.
As with most Sylvania Locktal valves the gettering is on the inside of the dome and the lettering is on the outside. The glass thickness and the reflection off the inner surface give the effect of a double print.
The wide glass tube envelope is 28 mm in diameter, and excluding the B8B base pins is 55 mm tall.
Reference: Datasheet. Type 7L7 was first introduced in 1940.