Yashica ML 20mm f/3.3

Hello and Welcome to the forum, luca ...

You are correct in calling these rare. I don't know if there any firm numbers or even reasonable guesses on how many were made, but these and a coup[e of it's siblings were all released in low numbers. The ML line is the last in the series of these ultra-wides, and came in C/Y bayonet mounts - as 20mm and later as a  21mm. I'd presume that there are far fewer of the 20's, but either model won't show up for sale often in the same quantities you'll see for 24mm or 28mm lenses. There's also a 15mm ML, but it isn't a rectilinear, as it definitely does have the usual fish eye distortions.

Yashica 15mm test shots - raw and 'corrected' for distortion: testbed.lumiworx.com/#15496566479419

There are also Yashica 20mm lenses in M42 mount to consider if you're adapting everything for cine use. Unfortunately they can also be hard to find since they too are somewhat rare. The good news is that all of these ultra-wides come with very similar AR coatings, and as they are all similar in optical properties, you might be able to use a DS or DS-M model as a stand-in for an ML 20mm or 21mm, and be happy with the result. Side by side comparisons would show subtle differences, but they should all have the same general character.

The DS 20mm in M-42: lumiworx.com/albums/lenses/content/yashica-yashinon-ds-20mm-f33/
One 21mm ML: lumiworx.com/albums/lenses/content/yashica-ml-21mm-f35/ and #2: lumiworx.com/albums/lenses/content/yashica-ml-21mm-f3/

A group of test shots comparing the DS 20mm and the ML 21mm: testbed.lumiworx.com/#15416312347601

If you need to fill in the ultra-wide end of the set and and have trouble finding a Yashica 20/21mm, then you could always substitute a Contax Zeiss Distagon 18mm temporarily.

Ziess 18mm: lumiworx.com/albums/lenses/content/zeiss-distagon-18mm-f4/
Ultra-wides and Zeiss 18mm Test shots: lumiworx.com/tags/ultra-wide/content/

It's been a while since I've price shopped any of these, but not too long ago the price of the Yashica lenses were pushed up to nearly the same as a Zeiss 18mm. Rarity comes at a premium.

There are other bonuses with the ML 21mm lens, too. As the last of the ultra-wide ML series they benefit from some improvements that hadn't quite been fully addressed in earlier models. They act very much like a flat field lens and have the same resolution and sharpness from edge to edge and corner to corner with almost zero light falloff, so no vignetting or smearing either. The minimum focus distance is very short, and almost macro-like, so you can get in extremely close.

This was shot with the lens about 9-10 inches away from the plane of focus, and set at f/5.6 to give it a smooth out of focus area and bokeh. I'd guess that the front edge of the cushion was maybe 4-5 inches from the front element. lumiworx.com/albums/the-color-plaid/content/impossible-landscape/

I've had a few, and I've liked them all very much. They seem to be very consistent in quality in my case, as I've never found one that was less than excellent. You may have to be patient to find one though. They might all be hiding for a year or so, and then there could be 5 or 6 showing up within 2 weeks.