I just picked up an excellent vintage Brown & Sharpe No. 4 combination square protractor head and blade for $20. It was in good shape, just needed some cleaning and surface rust removal. Made in Rhode Island, it’s a high-quality, forged and hardened precision measurement tool. Not sure of the vintage.
These are great for transferring angles, measuring and marking at non-square (or square) angles along the length of the blade, and using the bubble level not just to check square, but determine the exact angle of out-of-square surfaces. The blade can flip to both edges on both sides, giving four different graduation sets.
Here’s a photo essay of my cleanup process.