I used to build lots of tiny steel MTB frames in the early to mid '90's before the big manufacturers decided that small sizes were worth stocking. This one is unusual in that it has S&S couplers and it is made for smaller profile tires , maxing out at 26x1.8".
The customer did a wise thing and went for a disc brake setup. I think this is the way to go on really small frames as there will be no issues with the cables or the heel room that rim brakes would present.
there are rack mounts front and rear so the bike could carry a pretty sizeable load.
Here are the couplers and there's even a front derailleur boss as the equipment on the bike will be a mix of Shimano Ultegra and XT components.