When was the linearizing coil introduced?

Looking at the patents:

SU890 (Russia, filing date 06/23/1921) – oscillator without a linearizing coil,
US1661058A (USA, filing date 12/05/1925) – oscillator without a linearizing coil.

In RCA and custom theremins for Clara and Lucie, the linearizing coil is already present.

It would be interesting to know when and under what circumstances Lev Thermen came up with this idea. In principle, it seems to merit its own separate patent.

"US1661058A (USA, filing date 12/05/1925) – oscillator without a linearizing coil."  - ILYA

Oh, so interesting to read that, thanks ILYA!  In it he's aiming for 500kHz to keep it out of the AM band, I'm going for 450kHz for similar reasons.  He also refers to the field as "electrostatic" which I feel is a misnomer since the field is AC.  It also seems he's buffering the oscillators before mixing, shades of the ESPE01?  Also mentioned are multiple pitch antennas for a polyphonic instrument.

I wish I could shed some light on when the linearizing coil was introduced, but I got nothing.  I'm guessing he sensed the non-linearity of the near field and decided a double resonance might help ameliorate it to some degree, and went down that path.  It seems he's almost part of the way there with the transformer coupling to the antenna.  I agree that it probably deserves a patent of its own.

How does one view SU890?  I couldn't find it.

I've uploaded the Russian version here (SU890rus.pdf).

I no longer remember where I downloaded it from. There used to be an excellent website, trmvox.ru, hosting a vast collection of documents and articles from various magazines. I managed to download quite a few of them before it went down.

Please note that in many publications about Lev Termen (Theremin), this patent is incorrectly cited as №780.

Here (SU890eng.pdf) is the machine translation into English.

"I no longer remember"

It seems it was the official website of Rospatent (the Russian Federal Service for Intellectual Property).
However, it now only displays a reference to the patent, without its full text:
https://www1.fips.ru/iiss/document.xhtml?faces-redirect=true&id=f10c1586e56c930be35eaef170880d19