That's right; that's the volume control at the antenna input...
. Input impedance matching something of a bear, maybe?
Just a simple RF Gain control, rather than the more conventional AF Gain control somewhere around the first
IF amplifier. (late edit, that should be
AF amplifier)
Impedance matching?
Don't over think these things, in normal use this receiver would have been connected to a random length of wire, the feed impedance of which would have varied considerably over the tuning range of the receiver.
Would a receiver expected to cause interference?
Regenerative receivers can, under certain conditions
We need to be careful of the terminology here.
There are "Regenerative receivers" and "Super Regenerative receivers".
The operation of the two types is somewhat different.
In the regenerative receiver, there is positive feedback which is adjustable.
To receive an AM signal, the regeneration (feedback) is advanced until the amplifier stage is not quite oscillating.
This will give the best sensitivity and tightest selectivity.
If the regeneration is advance too much, the stage will become an oscillator, and unless there is a buffer amplifier between the regenerative stage and the antenna, it will radiate quite nicely on the frequency to which it is tuned.
Making the regenerative stage oscillate is (was in the old days) a nice easy way of receiving CW (morse code) signals which are simply on/off keying of an unmodulated carrier.
A superregenerative receiver on the other hand is oscillating all the time, and being switched in/out of oscillation by a "Quench Oscillator".
Superregen receivers can be very sensitive for a simple circuit, and do radiate a lot of wideband noise.
JimB