This part has been documented in three variations. The earliest, DMS 1308, was introduced in 1914. It has only two transverse (crosswise) holes for axle rods, both passing completely through, and has three tapped holes for set screws. These tapped holes only pass through one side of the coupling - this is known as "single-tapped". See illustration 1; these four illustrations show both sides of four different variations of part 63. In 1917, a third transverse hole for axles was added, DMS 1309; this is shown in illustration 3. It is still single tapped. Double tapping was added throughout the Meccano range around 1927, and this version, DMS 1310, is shown in illustration 4.

The puzzle is illustration 2. This matches the form of illustration 1 and DMS 1308, but it is made from what appears to be a zinc alloy. All examples of this variation which I have seen also have setscrews, as shown, not grub screws, as on the other couplings. Others have reported this with setscrews. Is this zinc alloy version a Meccano part?

The zinc alloy version is tapped 5/32 BSW. It certainly seems to be a Meccano part. An interesting possibility: in December 1919, the Structo Company's metal construction system was purchased by the US Meccano company. Structo used cast zinc alloy gears, wheels, and pulleys - is it possible the US Meccano Company had Structo cast these parts? Zinc alloy couplings are also known outside of North America; see www.nzmeccano.com/Outfitsnickel.php and scroll to the Inventor's Outfits.