This faint and rare object is on my dream list since long, and finally I managed to record it during 5 nights (one of which was discarded as a whole, due to low quality) in February and March 2023. Despite low altitude and not good atmospheric conditions, the image came out pretty clear. The supernova remnant was extremely faint and required a lot of careful work to be amplified. I drizzled x2 to manage under-sampling, that lead to a 96 megapixel image. The combination of the Canon DSLR and the L-eNhance EOS Clip Filter keeps surprising me positively, picture after picture.

SNR G206.9 +2.3

Monoceros is full of interesting objects, some extremely popular, some less, as the one presented here, the SNR G206.9+02.3, that has been no-so-frequently imaged in the past. At the date of publishing (March 2023) there are only 3 pictures in AB of this object. It lays close the more known Monoceros Loop, SNR G205.5+00.5, that shows a larger and more evident ring. The red nebula LBN942 visible on the right side of our picture here belongs to the east portion of the Monoceros Loop. The drawing 1 here below shows the relative positioning of the two objects.

The distance of SNR G206.9+2.3 is inconsistent in the literature. This is partly due to the low intensity of the object, both in the optical and in the radio range. Graham et al. (1982) estimate 3 to 5 kpc (9800 ~16000 ly), while Odegard (1986) derives only 400 pc (~1300 ly). Clark & Caswell (1976) state 2.6 kpc (~8500 light years). As for the age, Leahey (1986) estimates it to be 60000 years old using X-ray measurements. (*)

The shape of this supernova remnant and the "Champagne Flow"

In the south eastern part it shows a pretty regular hemisphere (see drawing 2 below), while in the northern side it seems broken, that might be due to less resistance encountered in interstellar medium (something similar to the Sh2-224 Sampan Hat, with different result).
Or, different scenario, it could be that the object has a hourglass shape seen at an inclination... It would be interesting to hear your ideas about that! (I even tried to ask to the NASA APOD team, maybe they will answer!).

I chatted offline with @Gary Imm, who had already been drilling into this topic, and Gary tends to favor the first hypotheses, the one of differential resistance in ISM.
I also spoke with a friend, who is a professional astronomer, who confirmed this and specified that the rest of the supernova might be still immersed in the molecular cloud from which the star was born. The supernova remnant reached the edge of the molecular cloud at the top, but not at the bottom probably due to the motion of the star. This phenomenon is called in literature 'champagne flow' or 'blister'.


Drawing 1: position of supernova remnants SNR G206.9+2.3 and Monoceros Loop.


Drawing 2: main ridges visible in the SNR 206.9+2.3. The upper part has probably broken because of the Champagne Flow effect

(*) the paragraph about distance was excerpted and translated from https://forum.astronomie.de/threads/14-woche-der-supernovarest-g206-9-2-3-im-einhorn.286227/