Reddit mentions of Starrett C435-12-4R Combination Set, Cast Iron Heads, Satin Chrome Blade, 12", 4R Grad,

Amazon has 25% off the Starret 12" combo set

The "reversible" just means that the protractor can be rotated 180 degrees in either direction. That's why it has two "" marks, one for using it with the flat facing left, one for using with the flat facing right.

Amazon has pictures of both sides. Sounds like you got a real one to me.

No, the full set with the crappy cast heads is $220 on amazon, and the full set with a forged head is $328.90

Cut across its not hard to visualize a 45° angle, 90° is square, 45° is half of that straight across or diagonal. They have combination squares at any place that sells tools really. This will take care of any degrees of measurement, square or 45°, center of tubing or any varying degrees of measurement its called a combination square I have a starret set but any cheap set will do. I use it for metal fabrication, I have to fabricate a lot of stuff by blueprints from scratch at my job. https://www.amazon.com/Starrett-C435-12-4R-Combination-Heads-Chrome/dp/B000KUA2OO/ref=mp_s_a_1_11?ie=UTF8&qid=1549104729&sr=8-11&pi=AC_SX236_SY340_FMwebp_QL65&keywords=starrett+combination+square&dpPl=1&dpID=41tZuP1%2BF9L&ref=plSrch