Are you sure this is the way the joists are designated? I have a copy of the Joist historic manual from SJI and I do not see aything in there designated as such.
There was an 'S' series from the 20's through 1960. Makes me wonder if the 3rd number is really an 'S' not a '6'.
At 22" and 24" depths, my tables only shows '22S7', 22S8' and '24S8' joists.
Only other thought is that these are some kind of proprietary designation from some manufacturer.