Original Meccano early 56T version of part 27a discontinued in the 1920's with a single tapped hole for a grub screw (not included)
A used part showing signs of discolouration through age & use
A plastic multi-purpose gear with a brass centre.
Can be used only together, but at any angle.
Can also be belt driven like a pulley
Buy as a pair and save approx 1/3