Rectangular tubes are much easier to work with than round tubes: No tube blocks required for hole saw notching, and the stay miter is a flat cut. That little tube insert is for looks but it probably helps stiffen up the mono-stay a little too. This type of stay would work well with removable chainstay brakes as the square tube is low profile and doesn't present any clearance issues (like a 1.5" round mono-stay does)
This is a trails 22" wheeler that will be for sale once I get around to coating it. Here are the numbers:
21" TT
74.25* HA
13.75" CS
68mm Mid BB
Straight gauge DT/ Supertherm TT
12.6-12.9" BB height (+1.55")