What you need to know about 12X20 is that there never was, and there is not now, any standard. In the old days Korona and F&S holders were not interchangeable and most cameras were built and fitted to the holders as a one-off operation. There was some standardization by both Korona and F&S but in spite of that fact you will sill find a wide variance in size of critical dimensions between backs and holders, even those made by the same builder.
The same is pretty much true today. There is no standard. The dimension of S&S, Lotus and AWB are all slightly different. The difference is usually in the location of the rib-lock, since width is close to the same with the three types. In practice what is done today is that camera maker builds cameras of this size to fit a specific make holder. In most cases the only way you can use several different holders in the same back to is rout a different groove in the back for the rib-lock. This is a critical issue because it is the rib-lock that make the holder light tight with the back. In fact, I estimate that at last 50% of the problems experienced by new users of ULF equipment is light fogging from improper fit of the back and holder.