The distance between the sensor and the last lens element, if I'm not mistaken. With the RF flange distance being 20mm, this means the lens sticks out beyond the mount quite a bit, which might mean this is not a practical design.
It's practical enough that Canon has done it before. Since there is no mirror with R series cameras, lens designs that project close to the image plane are possible, just as they were for rangefinders. Canon's 19mm f3.5 was a 1960s rangefinder ultra-wide angle lens that projected deep into the body beyond the mount. See https://www.canonrangefinder.org/Canon_19mm.htm The new patent shows optical features with some similarities to these early "symmetrical" designs.