That is what I have heard as well. What is funny my instructor from a bazillion years ago (I am old guy btw) called them Fitting Shallow and Fitting Deep....oh well, go figure, same as calling a 4 square box a 1900 which is an actual part number from one of the first manufactures of the box.
Well I found this old yellowed book of mine. Tom Henry...my first instructor from "1980 sumpin"..FD boxes as we know are malleable iron, aluminum for use with RMC or IMC with theaded entries and are called Fitting Deep or FItting Shallow.......ie. FD and FS
My guess, with absolutely nothing to back it up, (never stopped me before ) is Fitting Deep and Fitting Shallow, because most of them already have threaded hubs built in.