I could get in trouble here so I'm going to tread lightly and preface this by saying that everything I'm about to tell you is purely speculation based on my observations.
Nailbender's drop ins are quite standard in terms of their design and construction. I believe he uses a pill/spring "kit" that he probably gets in bulk from somewhere. He also orders his reflectors from somewhere else (and they thread on to the pill), as well as his drivers. He basically solders and assembles everything according to your specifications, adds some thermal potting, tests it and sends it your way. He does a great job and he uses good stuff but I don't think he's making anything from scratch.
Malkoff's drop ins are a horse of a different color. The reflector was designed by another very reputable flashlight maker around here (McGizmo) and it's bonded inside of a very hefty brass heat sink/body that is machined specifically to the specifications of the drop in. The driver (I'll be honest I don't know what is used) is soldered inside the brass module and the tail end is filled with some kind of epoxy. It's just more customized and clearly has a higher cost of raw materials and assembly.
You can see what I'm talking about here:
(Image courtesy of Oveready)
Image courtesy of Lighthound.
Also note that the M91AW just came out. It may drop after a while.
To answer your other question, yes this will fit a Surefire 6P. It was designed to do exactly that.