This is an oblique response to the question on the accuracy of Starrett Combo Squares v. other manufacturers.
Although the Starrett is a first rate combo square, it still cannot compete with a good quality try square in terms of repeatable accuracy over the life of the tool, due to that fact that it has moving parts that are subject to wear.
The Marples 9" "Shockproof" (no longer manufactured - r.i.p.) was manufactured to conform to the British Standard 3322 of +- 0.01mm per cm of blade length. (El Sauro has its mate and can check it against his Athol Collection).
The Ulmia has a stated tolerance of +- 0.10mm over the length of the
35cm blade.I've used both of these try squares for a number of years and reach for one of them to check squareness of cut rather than for the Starrett Combo Square.
For machinery setup I use a Starrett No.20 Engineers Square.
I periodically check these squares, using a Starrett No.380 Machinist's Straightedge as a reference and have never had a need to adjust them.
The point, as it relates to the question of the OP, is that it is cheaper to manufacture a dead on try square than a dead on combo square and the try square provides a third, economical option to the Starrett v. Other Manufacturers dilemma. (remember, even god never made a square johnson)
I fear that the preferential usage of the combo square over the try square is a direct result of the influence of Brother Norm and his Carpenterish ways.
We should seek to overcome this influence, insofar as we aspire to the heights of craftsmanship implied by the search for accuracy that must be measured in angstrom units and its application to a material whose deformities are measured in fractional inches.
I remain, tongue very much in cheek,
Y.O.B. (yer obstinate bastige)
Regards, Tom Thomas J. Watson-Cabinetmaker Gulph Mills, Pennsylvania