But ADI is an ODM and they doesn't sell at retail. Retail choices are an RCC-VE from Netgate or an SG-6840 from pfSense. I considered both and (to support pfSense directly) I went with the pfSense device although the Netgate is a little less expensive.
However, neither Netgate nor pfSense produce any detailed hardware description, as might be needed to install additional components such as an SSD or wireless board. Users have to rely on the ADI documentation plus additional product specific documentation from the OEM or retail vendor. The latter should describe all product differences from the former, but doesn't in this case.
I was trying to install an SSD in the SG6840, which should be an utterly trivial task, but it doesn't seem to be possible with the current pfSense release. It was only possible with the development version that you linked to in another forum thread. And very many thanks for that link :)
Frankly, if one of my product managers had permitted First Customer Ship of hardware with incomplete documentation or without the availability of fully tested software needed by customers then he (and I) would have some explaining to do. The sales folks who interface with customers would want our heads prominently displayed on poles at the building entrance as a warning. I'm only half joking, they probably would be pleased with that remedy.